AFAIC when it comes to Mac Mini late 2012, there's problems under the hood AND there's not much happening in the "hood" as I'm not finding the solutions to the problems. I suspect that whilst the Mini was eligible for the OS update, it really can't handle it. I would've liked to have known that before going down this path. I've been told that I need to "buy 3rd party applications to replace the Apple bundled apps because they don't work so well".
This is a clean install, 16GB RAM. No disk problems (apart from not being SSD). I've reset NVPRAM, SMC and no 3rd party apps have been installed - YET ( I shudder to think what might happen – I need to reinstall MAMP etc. ANYONE PLEASE tell me if you think I should step backwards to Sierra/HighSierra).
The current issues I haven't resolved:
bluetooth keyboard typing backwards
TextEdit takes 30-secs to perform a cut and paste.
Mail app can't/won't read by previous mailbox database and so I have to buy a 3rd party app to transfer the contents.
After Mojave what I've got is a giant desktop iPad without a touch screen.
have you tried reinstalling the OS? also, your drive could have damaged sectors. and you can't import your older mail? have you tried importing as mbox? which bluetooth keyboard? if it's 3rd-party, check with the developer (for example, there may be a new driver for it). the whole point of the apple apps is that they do work well with the OS... 3rd-party apps won't fix those issues.
you can try setting up a 'test' admin account, log into that, see if the keyboard works, textedit works; that would, at least, tell you that the OS isn't the problem, it's something in your account (ie a preference issue, etc).
seriously, if an OS reinstall doesn't help, and you're up for it, get something like carbon copy cloner and a 7200rpm external drive, and... clone your mac, then reboot into the external drive, and see how things go.
good luck with this!