My ds3 was potty trained 6 months ago but only started using the loo regularly about a month ago. I have seat inserts for both loos but that didn't seem to encourage him to use them. One is squishy and soft because I bought it years ago for ds1 because I worried he might not like sitting on a hard seat, but TBH, our other seat, which is hard, is better than the squishy one, mainly because it has a bump at the front which helps to keep the wee in.
Anyway, the way I got ds3 using the loo instead of the potty was, firstly, only having one potty which I kept downstiars, so if he was upstairs and needed the loo, he had no choice about using the toilet.
Downstairs, he still used the potty though, so in the end I had to physically remove the potty from the downstairs bathroom so he had no choice but to use the toilet there too. He kept asking for the potty of course, but I just said "No, use the toilet" and mainly there was no fuss. The potty is still around at the moment because it is useful if my other children are on the loo and ds3 needs to go. Also, he still can't get himself on the toilet, so if I am not around to put him on, he gets the potty out of the cupboard and uses it then.
But progress has definitely been made. (BTW, he is just 3).