It’s not really about the hardware, is it?

It's about everything, that's the point

The option you mentioned won’t enable an alternative app store, it won’t enable access to android app emulators

I don't see how that would help in any way to secure the device if you don't trust Apple.

The level of trust iPhone users give to appeal is wildly higher that what android users that tweak their phones give the manufacturers.

You either trust a company or you don't. There is no grey area. If you don't control the whole thing, you don't control anything at all. A custom ROM on your Android device is not going to do anything to prevent a firmware or hardware level backdoor. Your custom ROM doesn't improve security, on the contrary. If you unlock the bootloader you break the chain of trust and all bets are off.