If possible, not JS/browser emulator, I would like to see as "seamless" integration as possible, ideally running UNIX or BSD from the regular Linux terminal
If you find SIMH, look for a "software" folder nearby, which contains several operating systems. Some of them,
like Unix, may be tricky to install unless you have an installer's guide. BitSavers.org is a good site for documentation and several items of system software.
There is the pidp-11. I believe it does use a modified version of simh behind the scenes though.