Some throughts on your comments:

Yes, there is less droid humour in Empire (except at the very end, when R2-D2 is reassembling C-3PO), but as you say, that's appropriate. The real humour in Empire comes from the Han/Leia screwball rom-com banter ("Would it help if I got out and pushed?"), and it's a perfect counterpoint to the darker stuff.

From a visual effects perspective, I do agree the use of CGI tweaks has dated the films to late 1990s when up to that point they had seemed timeless due to the reliance on practical effects. That said, I do like the additional material in the ending celebration, which is where the bulk of the additions were in this film (the more egregious addition is in Star Wars, with the entirely redundant Jabba scene).

It's not 100 percent clear whether Luke returned to Dagobah between The Empire Strikes Back and Return of the Jedi, but I'm fairly certain he didn't, or he'd have asked about Darth Vader before. That said, I think he must have done a lot of additional Jedi training without Yoda to get into the kind of shape he's in at the start of Return of the Jedi (Luke is much stronger in this film than previously).