Does Tali have less romantic moments because she joins you later? I don' believe that. Anyway Garrus is a squad mate throughout the whole trilogy but his romance can only start in ME2 whereas Liara isn't a squad mate in ME2 (except in LOTSB) but she has romantic content in in very game.
There is less romantic content for ME2 LIs except for Garrus and Tali but I would say that Thane and Jacob are the worst and in Citadel DLC you can organize funeral for Thane. Not very romantic...
It's true that male Shepards didn't have male options in earlier games but it seems that it has changed. Anyway if Thane and Jacob would have been male only I wouldn't have minded because I'm interested in romancing male LIs so it doesn't matter what my PC's gender is. Although I prefer playing as femaIe PC. Anyway I would have romanced Thane even if he would have been option to male only because I liked his romance. It's just I don't what that kind of male LI in the near future. Not to mention Jacob.
You're right. I don't think Garrus has more romantic content than Tali but I do know that it annoys me that I get Kadan on Mars only to lose him for what seems like half the game. I really lucked out with Kaidan because not only is he my favourite romance in any game but I love having him along as a squaddie too. Even if it's not romantic content, I wish I could have taken him to cure the Genophage or liberate Grissom Academy. I imagine Tali-mancers are a little peeved about that too especially as the same thing happened in ME2: Garrus can be your first recruit after Freedom's Progress whereas you have to wait until after Horizon for Tali.
I also agree with you about how much love Liara got as both an LI and as a squadmate. She got her own 3 (large) mission DLC and a love scene in ME2 and she wasn't even a standard squadmate. In ME3, you can take her on literally EVERY mission in the game (where it's possible to select squadmates) and she is required on four (Mars, Palaven, Eden Prime, Thessia). Most squadmates have only one compulsory mission each (Vega has a tiny bit of Mars + Palaven and Tali has Geth Dreadnought + Rannoch). She is also the only squadmate in the series that you absolutely must recruit in more than one game.
This is all sounding like I hate Liara and I don't at all but I will freely admit that I'm jealous of the Liara-mancers that get to spend so much time with her across the trilogy. If I were to list/rate the LIs quickly based on both how much time you spend with them + romance content across the tilogy, it would look something like:
- Liara
- Garrus
- Tali
- Kaidan (straight only - beats Ash because of Citadel DLC date)
- Ashley
- Miranda
- Jack
- Kaidan (gay)
- Thane
- Jacob (romance content in 3 doesn't count so it offsets being first squadmate in ME2)
- Steve (Beats Sam because as the shuttle pilot he has a Leviathan romance dialogue plus goodbye video chat on Earth)
- Samantha
Kelly/Samara not included.
I'm not sure what this all means except I think I've concluded that Jacob is mathematically (haha!) the weakest squad romance and would probably be worse than Steve/Samantha if not for the fact he's literally the first person we meet face-to-face in ME2. Also in my totally [not in any way] scientific analysis, I've concluded Liara's romance to be be the most robust. Of course none of this takes into account the quality of the LI and I have spent waaaay too much time thinking about this 
I will stop.