All that sounds great. Your tank is big enough and you aren't rushing the cucumber intro. Sounds like a plan! Again, I would let detritus pile up, so it has a 'backlog' of detritus buildup, to ensure it has enough. Sinking veggie-foods can also be added to supplement.
The sand swap sounds tricky. To help speed up the transition, consider mixing some of your old gravel into the new sand. You very well could go into a cycle again, so you'll want to test for ammonia, nitrite and nitrate to keep in touch with conditions, and change out some water where necessary.
Good luck kf1978 and welcome to RC!