God bless you Da Vinci, you gender-bending rocket-armed weirdo.

All event will always have easy acces to some ascension material

I think the next loot box event should be the Nero Fest event, around late September, according to the JP schedule.

Davinci is hands down the second best for farming acension mats and exp(after xmas).

To answer your question - its complicated.

The upcoming event fate-acell zero event is almost as grindy as davinci event, and gives you alot of ascension mats as well, due to its jam packed shop. However, it does not have a loot box style currency(it does have a mona lisa style one and an event shop), instead shoving SQ and summoning tickets(I recall at least 8 tickets and 10 SQ) and apples down your throat.

As for the next mat farming event, Its hard to tell, I recall that nero fest was ok for grinding, but the xmas events should contain lootbox systems as well.