My understanding is there's little difference in power between the 500 and 520 so if you don't have use for the dump bed, and given Honda has reduced the 500's price, were I you I would just buy another 500.
I notice your P500 has 11,000 miles which is impressive to me, properly maintained how long are these engines good for. The local dealer had a used 2018 P500 with 4,000 miles which I thought was a lot.
In my case I use the dump bed a lot so the P500 not having one, along with no rear differential, took it off my radar. Now that there is a bed, even though no differential, I decided to replace my trusty Rancher 400 (which didn't have a dif either) with a 520.