I had a Sundowner on my previous FXBB and it was very comfortable. Did a few times 450+ miles without any issue,

 

Might consider doing a tailbone relief (pocket hollow below your tailbone).
You could remove some staples to gain access and DIY or most any upholstery shop should be able to help you out.

 

Just want to say: THAT is a beautiful bike. Of course I'm biased because my 2019 has the same paint scheme! Seat looks very cool. I know it's not the topic of this thread, but what handlebars are those? Mine has 12" Chubby's mini apes on it, which are cool, but I like your lower profile bars.

EDIT: Sorry - did that wrong. Was referring to the Billiard Blue and White heritage with the old fashioned solo seat.

 

I post this repeatedly whenever I see a seat thread. Have you considered having your stock seat reworked? I had mine done by C&C and could not be happier. They rework a seat to your specifications.

 

Mean City Seats will re-work your seat. They do good work at a good price. Had 3 different seats modified by them.

 

I installed the Mustang Wide Touring Seat right after I bought the bike.
I had one on my previous bike and loved it.
I've put over +55,000 miles on it and still love it.