That’s a very clever life hack to use shaft coupler as a starter. Another part that can be used, if you don’t have couplers, is an insert for a 12T HS pinion.
But if you feel that it takes excessive force to install the inserts, you may be risking putting extra strain on the plastic of the gear and they may end up fracturing under the load. To avoid that, I would try to file off the sharp edges of the metal inserts (very very slightly).
Another trick to deal with the very tight metal inserts is a special insert puller tool, which could be easily made from an old shaft with a grinder:
First, you insert its head into the square hole, turn it 45 deg, then pull - saves a lot of headache.