I don't own a Switch, so there's only so much feedback I can give here, but from the sound of it the issue is memory (RAM) usage. Obviously, this will be fairly limited on the Switch, and the game's engine can be very demanding.
Inventory will require more and more RAM during play the more and more you save and reload the game. There's a similar situation that can arise on PC, where inventory situations can start to arise after around 2,500 save/reload combos (which is, admittedly, a pretty insane amount. A playthrough of the entire game and NG+ will probably only generate a few hundred of these instances.) The item ID tags start to get very big, and when they become too big, the game can't assign the needed IDs. Hence, inventory lag --> weirdness --> corruption --> crash. This can be
prevented, but it is very hard to
fix.
Recommendations:
1.) Ensure you have not filled up the storage space on your Switch. Ideally, you want to try to keep about 20% of the available storage free at all times. If the game has trouble swapping data when it needs to, it can cause issues. It needs free space to do this.
2.) Don't overload your inventory. Especially carrying multiple copies of similar items (like many "Steel Sword"s) will exacerbate the problem described above. (Truly, there's almost no reason to horde items in the game. Most gear will become quickly obsolete and need to be replaced anyway. Once decoctions and potions are created, you don't need the reagents for anything. Aside from early on, there's no real reason to sell off weapons an junk loot worth only coppers, so just grab the expensive stuff.)
3.) Don't keep more than ~10 of the same Runestones or Glyphs in your inventory -- use them or stash them. There's a bug that can arise when you have too many of the same Rune/Glyph in Geralt's active inventory. It doesn't happen all the time, but it can be an issue if it does. (Like: a "Bye-bye, saved game!" issue.) As far as I know, the cause of this was never found. It seems to happen completely randomly.
4.) Don't fight the reality. If you notice the game starts lagging in a merchant screen, or acting funny -- exit. Save your progress in a new slot. Buy things 2-3 items at a time.
5.) Try disabling online connectivity, Friends features, social / marketplace stuff, etc. In other words, try to free up as much of the Switch's resources as you can, and see if that allows you to progress through the trouble areas.
It's a really ambitious title to have brought to a handheld. If you encounter something that reliably breaks the game -- don't do that.
(But
do send the info into CDPR Support!)