It is not a serious fault for user
who/whom/whose/which/that don't use English as a first language. For a long time, since the day I left school, I have not yet check basic English grammar. Thank you for the warning.
Minor faults do not change average post quality or seriously change a good post to a shit one. You can write a perfect sentence, in vocabulary and grammar, but it neither does make your post looks helpful nor creative.
Moreover, the length of your post and the quality of your English are only minor factors. In trying to write a quality post worthy of merit, you should offer new ideas, personal experiences, or perspectives that other forum users will actually find new and interesting.
I search and think that this one is great to start to learn how to use who/whom correctly:
https://www.grammarly.com/blog/who-vs-whom-its-not-as-complicated-as-you-might-think/
Among your applied posts, I only like
that one