Is there an easy way to limit user bandwidth usage to say 10kpbs? Other users should not be affected. The best I've found is tc, but it seems way too complicated to do such a simple thing.
tc is the correct answer to your configuration needs.
For you iptables rule just change --uid-owner to the uid of the user you wish to limit, documentation says you can use the users name instead of UID, have not tested that.
FYI even though you do not need an HTB I would recommend it based on the available options and what speed you are limiting to.