These are all variations of the Linux kernel, as in they are compiled with different options or components. In this case we're talking about different schedulers, which is the part of the kernel which decides what tasks are executed, when and for how long, etc.

I guess you're getting to the point in security where you really should consider the cost/benefit of safety vs convenience. Do you really want or need to have an immutable system? While there's obviously an argument to be made about the security benefits on those distros, I'd say that they're mainly made for CI/CD, cloud environments etc, and probably not something you want to put in a laptop and use as a daily driver. Your laptop is likely already more secure than 99% of other laptops, and in the end all you need to not get malware are a firewall and common sense if you're not an exposed entity.

I would say not in a way that makes sense, there may be hacky workarounds like setting nice priorities or messing around with scheduling, but there's no way around hardware limitations. The Pi's CPU, RAM, and IO bandwidth are what they are, and you need overhead to guarantee "snappiness"

What I meant is that overloading the CPU on a Raspberry running pi-hole will make the whole network misbehave and timeout, until DNS requests are able to be serviced again. But since they're not doing that it should be fine :)

It's probably going to be ok to scope them out and experiment a bit, but I doubt you'll get enough performance and stability to run it as production. Paperless' OCR is quite heavy on the CPU - iirc you can disable it but then you lose half of what makes it useful, and Nextcloud also does some processing to files that are uploaded to it. Since you are not running pi-hole or other latency-sensitive services it will probably be fine, it will just get sluggish while it processes uploads.