Didn’t quite follow 3DS emulation development, so correct me if I’m wrong, but it would seem mostly done. Only StreetPass and multiplayer seem to be a question mark currently.

Afaik, without power being a concern, pretty much anything until the PS3 that ever got an emulator for it, no matter whichever “host system” (borrowing VM thermology) it got released for, can be emulated on modern computers and systems.

Biggest caveats I can think of would be the options available, and how to run them.

For example, I try to avoid Retroarch when possible, since, to me, too many systems in one interface are a limiting factor, but the only stand-alone emulator that can decently run (imo) the Metal Slug games, of which I love, was some old Windows build of an arcade emulator, so I have to run it within Wine. Similarly, if you wish to run Java Phone games, afaik, you need to run them on a Java Phone emulator for the PSP within PPSSPP.

And on another example, PC-98 emulation is usually accessible only through Retroarch, but it doesn’t seem to be able to mount multiple disks at the same time, and some games need that, so, from what I could find, either you need to figure out how to use DOSBox-X as a PC-98 machine, or you need to figure out how to compile Neko Project II Kai for recent systems.

Old PC games that require Windows’ hardware acceleration and/or 3D libraries may also be problematic to run due to VMs’ development for old system being rather slow. Android also seems to be finicky, with either emulators being full of ads, privacy issues, moody compatibility with proper VM softwares, or taking a comically large space in Android SDK’s in-built emulator.

And progress on emulators for newer systems such as PS Vita, PS4 and Xbox One are slow, progress for Switch appears to be halted thanks to Nintendo, and heard iOS emulation is possible but it is still elusive to me.

So, to sum up, most of the times, even if with varying results, from what I tested and from what I saw being reported, most systems can be run, but may take some case-by-case setting up and testing.

If you want to gradually move away from closed and/or paid software, afaik, the only way is through unlocking the bootloader and uninstalling programs through there.

Alternatively, there is the nuclear option, to replace the whole system, and start from zero with a distro as close as possible from AOSP. Worth noting it also requires unlocking the bootloader.