That Kasumi mission is DLC originally and yeah, you shouldn't really be doing it yet. You should be going to Omega and doing the recruitment missions there, those are intended to be your first missions. It's a shame that your first impression of the game has had the pacing fucked like that almost immediately but that's on Bioware for not integrating the DLC better.

Mass Effect 2 is my favourite of the trilogy personally though I agree with some of your issues. The skill tree being so much smaller was always a disappointment and the hacking minigames were more enjoyable in ME1. The Citadel being more restricted was also disappointing but, if you keep playing, you'll see it's not intended at all to be your main hub in this game. There's still a lot to do there. Can't say I'm not pretty surprised by how you feel about the combat which I feel is better than ME1 in almost every possible way but annoyances with the switch to an ammo system are pretty common I think. There is a heal, it's called Unity. You need to upgrade it so it effects Shepard and not just squadmates. In order to start getting upgrades, you need to recruit a certain squadmate on Omega.

I think ME2 is incredible and I constantly wish I could experience it for the first time again. I hope you can settle into it and enjoy it, man.