I am testing repairs repairs -inc -par and I can see that in all my nodes the numbers of sstables explode to 5k from 5 sstables.
Make sure your version is higher than 2.1.12, 2.2.4, 3.0.1, 3.1 to avoid this (and you are indeed facing CASSANDRA-10422).
I am not sure you are facing this though, as you don't seem to be using subranges (nodetool repair -st and -et options)
It is anyway to run repairs incrementals but not -par ?
I know that is it not possible to run sequential repair with incremental repair at the same time.
"Incremental repairs can be opted into via the -inc option to nodetool repair. This is compatible with both sequential and parallel (-par) repair, e.g., bin/nodetool -par -inc <ks> <cf>."
So you should be able to remove -par. Not sure this will solve your issue though.
Did you respect this process to migrate to incremental repairs?
The Last Pickle