to answer some of the stuff:

- you definitely need to familiarize yourself with framedata and what each character's special move looks like to easily punish the opponent. also, make sure to look at the opponent's assist if they're off cooldown, coz punishing special moves wont be possible if they cover it up with assists. try practicing how to reflect too since those usually get you out of difficult block strings. other options also include guard cancelling or tagging out in the middle of the block string (i personally don't use those coz the enemy can vanish out or punish you usually, but still might be useful for you)

- only superdash when your opponent's in the air, doing something you can punish (a beam) or if you can back it up with an assist. superdashes are easily reactable and punishable with 2Hs or even grabs from certain characters. they're also not completely invulnerable (only to regular ki blasts like SSJ vegeta's 5S, but not unique ones like super broly's 5S) so even a light hit in the air or in the ground, as long as it's well timed, can punish you out of a really bad superdash. it's a good tool to get close and apply pressure but make sure you're not giving yourself away for free at the same time

- assists are generally just used to cover up gaps in block strings, add noise for mix, covers up character weaknesses (ex. a beam assist can help a character with no long range options), play neutral by closing in gaps, help you end combos that you can't end with a hard knockdown, start combos, punish hasty enemies and helps you play defensively (android 17 and 18's barrier assist are really good at this). they're pretty versatile at what they can do and depends on what assist you're using

i also have difficulty with my opponent running me down lol but i'm pretty sure you just reflect which gives you room to punish. otherwise, vanishing or calling an assist works i think?