Sacrificing a bishop so early. Eh not a good move
You're not sacrificing a Bishop. You are trading it for the knight to wreck the pawn structure in similar fashion to the Trompowsky Attack.
A number of GMs played the Exchange Ruy Lopez, including the nut job Bobby Fischer.
Giving up the Bishop for the Knight is not the issue. 5.b3 though is useless and the proof shows there with Black scoring an overall 72% after 5.b3 and winning 57% of all games.
5.b3 just goes to show that White is clueless what he is doing. White should castle here, and follow up with either 6.d3 or 6.d4, depending on which line Black plays, like 6.d3 against 5...Bg4 and 6.d4 against 5...f6. If Black's choice allows for the early d4, White should play it before Black gets in ...c5. White wants to trade that d-pawn for Black's e-pawn, creating a 4-on-3 majority on both sides of the board. White's is a clean 4-on-3 on the kingside, Black's is a crippled 4-on-3 on the queenside. White should not be voluntarily moving his queenside pawns. He should only move a queenside pawn when forced to, or after Black has committed to moving his such that Black can never create a passer. Best he will get is a trade where White maybe gets rid of the b-pawns and Black is left with a and double-c vs a and c.
All 5.b3 does is show that White parroted a few moves in the exchange Ruy, but clearly has zero actual understanding of the opening or what the point is behind it. Ideally, White wants to trade d-pawn for e-pawn and get down to a king and pawn ending, which would almost certainly win for White if achieved.
You do lose your bishop pair with 4. Bxc6 (and bishop pairs are powerful).
But I do agree that 5. b3 isn't a good move in and of itself if White doesn't know what they're doing.
queenside flank moves are very rarely harmonious with king pawn openings (that is e4 with b3, or e5 and b6). Usually its because they are too slow and leave holes on f4
it's definitely slow, but can you explain how it leaves holes on f4? i'm obviously not as knowledgeable about openings as you are, so i'd be curious to know.
if a knight goes on nh5 threatening nf4 how will you stop it? g3 weakens key squares on your kingside. Normally this isnt a big issue since the bishop can go bxf5 but since b3 means your bishop went to bb2, this option is not available (unless you waste even more time with bc1) hence you can say it weakens the dark squares around your kingside.