We went down to 3 days a week during the initial COVID lockdowns. Honestly, it was absolutely fantastic. I'd probably want to go to 3 or 4 days a week before I go into full retirement.
If I was at a baristaFIRE or leanFIRE level, I would absolutely consider it. Earlier in the savings process and the income reduction might not be worth it.
Not really, however I’m fortunate to get 33 days of PTO per year, so once my bank got near the limit of 40 days saved I was able to arrange for one day of PTO to be used every two week pay period. The extra day off really helps!