I adored The Adventure ReBirth when it first came out (partially because it was a return to the Classic style). Still enjoy it, though I'm not as into it as I was before. So, for your questions...
1.) I think it should be released in another collection that includes stuff like the SNES Dracula X for Switch.
2.) Overall, ReBirth is a mid-tier entry in the series as a whole. Of the 26 Castlevania games I've played through, I usually put it around #14 (it certainly is not any higher than #10). In terms of Classicvanias specifically, I rank it better than Legends, Chronicles/X68000, and the DXC remake, but that might be about it.
The Good: ReBirth does a decent job blending the original Game Boy titles with more NES and SNES sensibilities. Its use of multiple modes to result in multiple level layouts or gameplay styles (including a very functional motion-control mode) are commendable and add replay value. The density of each level having a strategic setup with alternate key-based paths is an inspired development. Knowing the best path through a level becomes crucial on harder mode settings. I like how more unique bosses were chosen on the whole, and the water wheel segment of Level 2 might be the coolest set piece in the game, which I'd like to see revisited in a future entry. Also, the return of a map screen was fun.
The Bad: The game does feel like it was made with a fairly limited budget, remixing assets, and it doesn’t utilize its namesake Game Boy entry to the fullest by any means. As a result, despite being enjoyable, it ends up feeling rather indistinct as a whole. The original The Adventure (and also Belmont's Revenge) have more iconic personality/charm overall. And for that matter, I know they wanted to put stairs back in, but ropes should have still been in it, considering it's supposed to be pulling from "The Adventure."
3.) The music is amazing. There are some really inspired choices for remixes. Unfortunately, it leaves way too many good Game Boy tunes off for a game that's supposed to be a Game Boy reimagining.
So, overall, I would have preferred if this was a combination of The Adventure and Belmont's Revenge, utilizing all the Game Boy ideas and bringing them up to Super Castlevania IV modernity, rather than picking and choosing a few token Game Boy elements and trying to make a new hodgepodge game out of them. The end result was fine for what it was, but it could have been a lot better.