I'd probably bump the threshold cost up to at least 30 based on the information you have.

You can make this change right in the middle of the day. It's very unlikely to cause issues other than changing which plans run in parallel, so, for the simpler plans, you'll get a recompile.

To measure the impact, I'd focus on those two plans that have a cost of 11 and 25 (assuming you set the threshold higher). See what their execution time is before and after you change the limit. As to the others, same deal, focus on execution times. Those cost estimates are so high, I don't doubt for a minute that those plans are, in all likelihood, good candidates for parallel execution. The other two, not so much.