I too find the seat uncomfortable after about 1 hour of riding and start squirming. I am planning a really long trip this summer and have been trying different things for comfort. Not sure if they would help with your specific problem, but I have done the following with varying degrees of success:
1. Installed highway pegs. This allows me to stretch out my legs occasionally and change the pressure points on my "bottom". This has done a lot to improve my comfort. I also have a knee that tends to pain after awhile and this fully addresses that issue as well. My highway pegs were an inexpensive ($100),but effective partial solution to comfort. I could not do without them now that I have them.
2. Purchased an airhawk inflatable seat pad at
https://airhawk.net/product-category/seat-cushions/bike-manufacturer/bmw/. It does help extend the time I can ride before I start squirming...maybe by as much as an hour...but I am not thrilled with the feeling of sitting on an "airbag". I am much happier when my bottom is firmly attached to the seat. I will use the airhawk sometimes but prefer to ride without it. It cost about 120$ CDN which is relatively cheap and helps to an extent with resolving my comfort issues, but I am not a huge fan.
3. Purchased MotoSkiveez underwear. See:
https://motoskiveez.com/ I was apprehensive to spend more than $60 on a friggin' pair of underwear. But as soon as I tried them I was amazed at the improvement in comfort. I am doing a trip of about 17,000 km this summer and will be taking two pairs of these; will keep alternating them and washing the other pair at night. When I took them out of the package I thought I had been duped/ripped off but I couldn't believe how well they improved my riding comfort. The Airhawk is pretty well redundant when wearing these. I would recommend these to any long distance rider. They take a little getting used to if you have never worn bicycle shorts or other underwear with padding, but you will love them when sitting in the saddle for hours on end.
4. Purchased a sheepskin seat cover from Egli Farms for under 100$. Can't comment on how well it works since this was a Christmas gift that I haven't used yet. But I am hoping that the sheepskin in combination with the motoskiveez and my highway pegs will be the answer.
PS - I did look at Sargent seat options and would love to try the seat/backrest comb. But buying the backrest forces you to buy the pillion as well (backrest attaches to pillion....how convenient for Sargent. But there is no way in hell I am paying almost $2000 CDN for a replacement seat.
Hope this helps you or other riders who are finding the stock seat "challenging".