Is it it bad to become a part-time student in my 4th year and do a 5th year in my undergrad if i plan on applying for grad school?
It seems from`

I reached out to a couple schools who are my top picks, and they told
me they have no problem with part-time studies,

that you've answered your own question.  Besides, you can't change the past, but only the future.  Therefore, my advice is to stop &quot;beating yourself up&quot;.  School can be difficult at the best of times, and things have certainly been hard during this past year of covid.
At one point in your question you talk about classes that you found challenging because of your background.  So, if those classes will be important to your future, it might make sense to do some independent reading on the topics that are causing you troubles.  Sometimes, revisiting something can make a world of difference.
Things have a way of working out.  Be sure to reach out to people in person, as well as over the internet.  You may find that a lot of successful people have also faced challenges.  Knowing that can help a lot in getting past hurdles.
Spring is coming.  So is vaccine. Things will start to get back to normal soon.  You've got years ahead of you.  Don't worry too much about the exact school you get into.  Find a spot that you think you'll enjoy, and plan to enjoy it and learn a lot.  Sometimes, just making the choice to enjoy something (even with challenges) can make a big difference.
Good luck.