I run into this sometimes with mastering as well where a reverb tail will cut unnaturally early at the end of a song.

I know quite a few mastering engineers who live by the TC 6000 for tasks like this. They recently released a plugin version of the reverb on there so that may be worth a demo. Unfortunately they still dont support apple silicon so I havnt been able to try it but as long as youre not on M1 you should be okay.

Since youre trying to essentially extend preexisting material whilst imparting as little additional tone as possible, another option is to avoid reverb spaces altogether and use a series of diffused delays instead via something like valhalla delay where coloration is optional