The thing about freeze is you don’t erase the original MIDI tracks (just keep them muted and the VSTi turned off), so if you want to go back and change something just make a new mixdown without the track you want to work on…

Only takes a minute or so, a lot cheaper than a new computer!

Which piano VSTi is giving you fits? You might want to check if you are tracking while more than one mic position is on (every extra mic tree doubles polyphony needs) and maybe back off on the sympathetic resonance samples until you render. Then there’s the issue of mixing the room mics with the close ones. Some of the top libraries can chew up 240 voices for just fairly normal playing, especially if you are heavy on the sustain pedal!