Grammar help

Hello, I find myself in a very peculiar situation tuat can't be described better than a guy on telegram did while talking on a group:

"Да он вообще прикольный. У него неплохое произношение, и относительно адекватный словарный запас, но на грамматику он хуй клал"

So yeah that's it. I am almost fluent in Russian but I have at most no idea of which case to use, and when, because I focused my studying entirely on vocabulary. I understand most of what is said to me, and I reply putting words I learnt together, and I get understood 90% of the times, but my grammar is absolutely shit. Ofc, with some experience I have acquired random info, like: -ого is a genitive singular and with "с", if it's plural you put -ами in the end, and similar things, but still...

Would you guys be able to provide any help? I'm NOT asking for textbooks or long lessons, solely because I don't have time for that sadly((... I learnt vocabulary very quickly with Anki, and I'd like to know whether there's something similar for grammar. I'd love your help.

P.S. I tried Duolingo but it's just NOT for me, I hate the green bird 😂.