I’m so sorry for the slow reply, I wasn’t getting most forum notifications since switching laptop, so am catching up on everything now.
It’s safe to truncate manually. But another way to do this is to set the max revisions to something very low like 3 (via General > Preferences). And then make a simple style change, which you can then revert. When you next load the revisions table you should see that most entries have disappeared and the table will be much smaller.
I hope that helps, and apologies again for the delay.