2014 imac running on external ssd

[Markverhyden]

It took about 10 seconds to open the System Preferences app and there wasn’t really anything else running. My MacTracker said it supports 10.14 and up.

LOL!! Talk about slow! Since it'll handle Mojave I'd go the external TB route. Of course that might work better than the original setup which will present it's own problems. If money's really tight you could dual boot the TB drive. Of course if it's out of warranty you can replace the internal drive and dual boot that. But the labor costs are going to be much higher.