How do i permanently remove all topics including their topics IDs so that their topic IDs can be reused by newer future topics?

It will not.

Don’t worry about the topic id. That is best. Whether the topic id is 2 or 20000 it doesn’t make much difference.

If you really want your topic IDs to be smaller then you can either restore a backup from before you created those topics or you could wipe your whole database and start again.