I worked at Amazon from 2006-2010 and received a lot of equity in the from RSUs (not ESPPs like the OP, but once you have the shares the source isn't as relevant).
Despite not being in tune with MMM or FIRE back then, I still followed the standard advice that has been echoed in this thread, which is to sell it all immediately. Concentration risk in your employer is never a good thing, etc...
So I sold...checks notes...over a thousand shares of AMZN in that timeframe for between $40 and $180. Not to mention the shares I gave up when I quit. In round numbers I would be about three million richer now if I had held on (minus taxes).
Now, this is a total hindsight is 20/20 thing. And who says I would have held on until today, this moment, at over $3000 a share? I probably would have bailed out around $1000 given that the stock was around $36 when I started. So the "three million dollars" thing is a pretty lame argument, but it still smarts a little bit.
Would I do anything different? Well I actually have the same situation now, just with my new employer. I get RSUs now, and I irrationally hold some of the stock for FOMO purposes. I have a 10b5-1 plan set up with Fidelity so that after each vesting event, after taxes are taken out, 75% of the remaining net shares are sold immediately. The other 25% I hold on to, and sell some at my discretion.
That question of "when to sell?" goes back to my initial story. If I had held on to any AMZN, when would I have sold it? Given that generally I am a buy-and-hold indexer, I don't have a lot of experience with these kinds of questions. Very recently our stock has skyrocketed as we have been pulling in record revenues during the pandemic. Up 4x since I started in 2017. So I have sold some of that 25% that I hold back. Honestly rationally, as everyone has said, I should sell all of it (especially at this all time high). But, well, FOMO. The shares I hold right now are about 3% of our net worth. So not a huge number, but enough that if it went up 10x from here I would be very happy to net that profit. I think I will probably try to maintain that 3% as a ceiling on my company holdings. Hopefully we keep going up up UP, and in retirement I am selling some along with our index funds at huge gains to fund our living expenses (but taxed at 0% because our total income is low enough).
So far I am happy with this plan. About 3 years in to my tenure here.