Well, to make a long story short I use the mod
BasketEquipmentNSFW (for science purposes and nothing else of course) and when I sell bags that collective is over 600 items the entire game struggle to load up (in this case) the druid vendor's stock when 600-1200+ items are on display in it. I'll test to "sell directly" and see if I can circumvent this issue. I'll load up a fresh save and report back.
EDIT: alas no. Selling directly still leaves items in the store i.e. I do full long "rests" in the camp to reset. I have tested to discard items in the river/drop on the groupnd and that does not work. Technically there are no ways in the current Baldur's Gate 3 to 100% get rid of items and this issue is something I experienced first hand in Cyberpunk 2077 where the savegame got corrupted (before the developers make an emergency patch) when we crafted too many items.
Some sort of "purging" of the vendors store (of user sold items) must be done automatically after X days. If it takes 20+ seconds for a vendor to open up it's store with 1600+ items, imagine what a XBOX S/X ore PS5 will do beside turning into a toaster. And let's not forget the size of the savegames. Another short term way to solve this issue might be if the "camp" have some sort of junk-bin that cleans up every rest.
Well, it might be wise to confront this problem in a beta than when the game is LIVE...
EDIT 2: Oh, I just discovered I could use the bag we summon with the necklace from the mod
BasketEquipmentNSFW that is active for 10 turns and then deletes itself to store all my junk in it (instead of selling them to a vendor). This is at least a partial fix for this issue.