Does apostates have to feel remorse for their non kufr sins in order to enter islam and it to be valid? In which books can i read more on this or listen to from scholars (eng subtitles) about this issue?

Ustadh AbdulAziz al-Haqqan
No, you can be a Muslim even if u didn't feel remorse for certain sins, but akhi be the best version of yourself, away from everything that is bad for you.