My issue with both a 9 and a double par max score is that this is primarily a net tournament, not gross. Sure in many cases the players in a given flight are all getting the same number of strokes on a given hole. But that’s not always the case. There are holes were some players are getting a stroke more than their competitors. If they are getting 2 strokes (for instance) and someone else in their flight is getting 1 stroke, the player getting 2 strokes will actually have a lower net score on the hole if they max out under either of those rules.
Most of my groups have gone to max net double bogey or max net triple bogey. I think max net triple for this tournament would seem reasonable. A player getting 2 strokes on a par 5 would max out at 10 (net 8).
Just my two cents. In the end it likely doesn’t alter the winning scores at all so that’s why the powers to be settled on an easier to remember gross score.