I did try the mod to increase party size, to run with six characters a while and get a feel for it. Besides the mod working in a kinda awkward roundabout way (you get "spells" that add or remove party slots) it simply felt a bit unnatural once I was playing six characters. Probably because I've already spent so many hours playing the game with four characters.
Anyway, what Tuco said above in May '22 (giggle) strikes true - with a party of six there is more room to experiment with less optimal classes and to push through a streak of bad RNG, without the entire encounter resulting in a wipe. With less characters you will often dance on a tightrope on higher difficulties, a single resisted spell or broken concentration can often mean disaster. I am still in awe of how quickly a fight can go from me feeling comfortably in control to a TPK, just from some nasty RNG. Just yesterday my run ended at the undead encounter in the Mountain Pass - a string of crit misses, failed saving throws, then the shepherds' turns. It was all over in a couple of rounds. (That encounter is horrible)
This is also my experience. With 6, you can afford to "miss" an action, so you can take a little bit more risk to make the encounter more amusing. Case in kind for me is vicious mockery. I love that spell, but it's not very effective, especially when you go to higher levels. So in a party of 4 I almost never used it. Yet in a party of 6 I can use it often, just for the "acoustic" effect and bringing joy to the game.
