I always open these threads to find out from the experts what they recommend to replace RPi as established, novice-level mini computers, but sadly I don't see any here yet
Orange Pi: Good documentation, but prices of newer models are not as affordable as previously
Radxa/Rock: Poor hardware support apparently
Pine64: Amazing hardware variety (phone, smartwatch, IP camera, soldering iron), but documentation can be hit or miss. Check the Pine64 wiki and search around for other documentation by community members
Khadas: Good documentation, and support directly from the hardware developers, but this comes at a cost
MilkV: Poor documentation - Ideal if you want to tinker
Libre Le Potato: Generally hear positive things about their hardware. Hundreds of these were used on a recent YT project in lieu of a Pi with great success, so may be worth a look.
Another thing to check would be Armbian's site - if something is supported by that distro, it might be worth taking a closer look at
A lot of the companies producing these "Pi killers" made them to survive the shortage, because their Pi accessories weren't selling. This means that generally they'll work great with the accessory, but support may be hit or miss outside of that.
I would lean towards Orange Pi personally, mainly due to cost and how long they've been around. Avoid the very early models as there were some overheating issues on a minority of the Allwinner chips - iirc their recent boards are using Rockchip instead.
It's not a perfect replacement because of increased cost, but there are plenty of sub $200 mini pc options these days. It's all included unlike the Pi which is still going to need a case, storage, and power supply. I bought one recently that blows away the Pi5 ,and it should because it cost more.