Hi,
Finally, I ended not having to delete my topics, I was able to do what I wanted to (not necessarily deleting them ; this could have been a solution but not the only one), see Migrate a phpBB3 forum to Discourse - #367 by Canapin, Migrate a phpBB3 forum to Discourse - #377 by Canapin and Migrate a phpBB3 forum to Discourse - #379 by Canapin