I resumed preparation after a sabbatical. I won't keep the post a long read. Start with the following:
1. Regular newspaper reading
2. Check last 5 years prelims papers and mains papers.
3. Focus on ethics if you have not attempted it before. Can choose any good online module or notes.
4. Complete static portion - it will give you the confidence that you know 50 percent of the syllabus.
Wrt mains, if you are attempting 2021, you have ample time to decode every topic of GS. You can join any online module or take up the notes of Vision or any XYZ coaching and start with them. The essence of what was asked earlier to what is now is not different. Just that they restrict you to fewer words.
Joining classes for all subjects is not the solution. See the syllabus, and identify your strengths and weakness - accordingly take a call for coaching.