My setting has technology more-or-less equivalent to Earth's 17th century, and a big chunk of my inspiration is Neal Stephenson's Baroque Cycle. The books detail the steps that led to the industrial revolution so my setting also has similar early tech, aided by magic of course.

(Airships, for example, use magic derived from Resilient Sphere to make their balloons supernaturally rigid and impermeable, then instead of filling it with a lifting gas they just evacuate all air from it. Their hulls look like solid wood but they are instead a honeycomb structure made of giant spider silk sandwiched between thin wooden veneers to keep the cold air out, and reinforced with the occasional mithral spar. The propulsion is purely magic though, the props are powered by aetherosiphon engines. There are some secret military projects aimed at creating a fully-pressurized heavier-than-air skyship that can actually fly over the taller mountain ranges; since their passenger compartment is not pressurized, a standard skyship's maximal cruising altitude is 3-3.5 kilometers while a trained military crew can maybe get up to 4.5 km.)

Hiring monodrones is usually cheaper. They are the simplest Modrons, and they would be perfectly willing to work for a Lawful king (the evil-good axis doesn't come into play) because it increases the amount of order in the multiverse. But every modron has Truesight.

Hell, maybe hire a whole team of modrons. Monodrones to stand watch at all ingresses, with orders of "raise an alarm if you see any disguised shapeshifter enter through that window / door / arrowslit / whatever", and duodrones with orders of "patrol the castle and raise an alarm if you see any disguised shapeshifter".

And that's why you as the DM can do passive skill checks (neé "taking a 10") for non-stressful situations. A routine landing is just 10 + ability mod (probably INT on a big plane with full FBW) + PB. It's only with 3 of the 4 engines down, the 4th on fire, the computers are fucked, you're trying to land the 747 on a dirt strip, and oh, there's a hurricane when you need to actually roll for it.

Though I'm also down with Esper's idea of every class having a limited reliable talent. So every character could pick one class skill at level 7 and one at level 14 in which they couldn't roll under a 10. The "expert" classes (rangers, rogues, bards, and artificers) would have additional picks at levels 3, 10, and 17 with full reliable talent being their capstone feature.

If it's a common item with a listed price, and you're in a city big enough to reasonably have that item in stock, just do your shopping "offline". Sometimes I even include a low-level Forge cleric in small towns so the party could do their sub-100gp item shopping. (In that case the cleric charges an extra 10% donation for the Forgetemple, which they will use to feed orphans, create farming equipment, etc...)

Plan 1: Raulothim's Psychic Lance. By the time I get to take that spell, I must have had at least one ASI so I'll just take metamagic adept for subtle spell. I just have to know the name of the target and be within 120 feet of them (240 if the second metamagic option I take is distant spell), and it's pretty close to a guaranteed kill vs. a commoner, no murder weapon, and nothing showing that I was there.

Plan 2: Clone. Claim that it's some revolutionary stem cell rejuvenation therapy and sell it to the super-rich. (Though I might make it fail accidentally if it's someone like Murdoch or Kissinger. Payment in advance, no guarantees, and I'll do it on international waters.)

Cantrips. Which also get more powerful based on character level.

Read the spell description.

Sure, 4d8 in itself is not much, of course. 4d8 per round as a bonus action for 10 rounds with a single spell slot, however, is a lot, especially at a level where a paladin only has second level smite slots. Which is why upcasting heat metal can be extremely powerful. (And it gives disadvantage for those 10 rounds, and there's no saving throw or attack roll involved. If you're wearing metal armor, it will fuck you up. And if you happen to be resistant or immune to fire, well, that bard 3 / sorc 4 might have transmuted spell.)

Casters also get proficiency up, that depends on total character level.

And higher spell slot levels still increase the character's damage output and specific utility. Some spells - such as Cure Wounds or Hold Person - are pretty much designed to be upcast, and some others (e.g. Heat Metal) are unexpectedly great. Someone with Bard 3 / Sorcerer 4 might only have second level spells, but a level 4 Heat Metal vs. a heavily-armored target deals 4d8 per turn without a saving throw for up to 10 turns.

Dunno. Arcane Tricksters tend to have a higher DEX modifier than INT (and then pick spells that don't care about the spellcasting modifier), so they are better off with Booming Blade (if stealth is already off the table) or Green Flame Blade unless they run into something that is resistant or immune to nonmagical damage at levels 3-4 and don't have magic weapons. Might work well with a maxed INT Arcane Trickster / Bladesinger build though.

Well, I don't think my players are here but I'll put it in spoiler tags anyway. Those playing in a world called Yphilios, don't expand this.

Hey, that's my meme!

(I don't mind, I reaped sufficient karma on the website we don't speak of, just funny coming across it in the wild.)

My players have been having an easy time with the enemies so far, thanks to literally half of the 6-player party being paladins with 20+ AC and a cleric with also 20+. Tomorrow they are fighting a really nasty homebrew fire giant lord with a special armor and shield, legendary actions, and legendary resistances. I gave them an out because defeat will just mean losing a bunch of newly-found magic items and being sworn to the service of said fire giant lord for 10 years, but we'll see how much of a reality check this will be for them.

Edit: they nuked him in less than two rounds. The giant got out a fireball as a legendary action, and rolled really high... except all but one of them saved (with +2 to saves because fucking paladins) and then all but one of them also took half damage (because fucking Ancients paladin). Then he managed to do two attacks which did pretty good damage against a paladin, but not enough to take her out. Then he got out a stomp attack as a legendary action but everybody made the DEX save because, again, fucking paladins so it was useless. Then he got out a Circle of Death... that everybody managed to save against because fucking paladins, and everybody took half damage above that because, again, fucking paladins, so it did a whopping 9 damage to a bunch of them which amounted to jack fucking shit. And meanwhile he got blasted with divine smites, including a critical one (with some assist from a hexblade's critical eldritch smite and a crit from a ranger with a powerful homebrew bow (about as powerful as a Dragon Wing bow) combined with Hunter's Mark and Hail of Thorns (edit2: I see one of my mistakes now, they are both concentration spells and I neglected to enforce this; will do the next time but it wasn't really the deciding factor)).

I don't know what I hate more, paladins or WotC's game design philosophy. Seems like the only way to challenge a level 5+ party in combat is to make it a horrible slog through a dozen enemies. Or, y'know, the FromSoftware design philosophy of slapping down an enemy with 20000 HP and an enormous weapon that deals 20d12 damage in a 90 foot cone or a 90 foot long 15 foot wide line.