iirc, the stages of splatoon 2 are bigger than that of splatoon 3 and have more height differences (i find that in a lot of s3 stages, i can see the entire map), so i suggest you familiarize yourself with the layouts through recon and check the map really often during matches.
before splatoon 3, gear sets full of main power up was super common, especially with bamboozlers and carbon rollers because they're originally weak weapons. don't know if that's still the case, but of course choose your battles carefully...!!! and if you plan to use it in your sets, you don't have to use so many. having main power up alongside other abilities tailored to your play style and weapon set will make you more powerful.
splatoon 2 specials also have a heavier emphasis on inking turf, rather than aggression, and they're all pretty easy to counter or run away from if your opponent doesn't surprise you. ink armor is bad. sting ray is hard to use, but super cool if you can master it (i never did lol).
i can't help much with octo expansion tho, sorry. regardless, i hope you enjoy splatoon 2, and that something here was helpful!!!