Your 260 watt FTP is not bad at all. I started seriously training a couple of years ago and lost about 20 pounds. My FTP didn’t break records but I got a lot faster. I’m 54. I peaked around 250/260 last summer. With a new power meter I’m at 230 today (indoor FTP) but that was after a summer without group rides or a lot of intensity and moving states.
I’ll tell you something about those fast 60 year olds. They are the guys that could run a 5 minute mile in high school without training. They are the guys that have been endurance athletes for decades and have always been in shape.
You don’t mention your volume other than SSB LV. There comes a time when you need to up the volume and frequency. Two winters ago I got a 20 watt bump in FTP from just doing base miles and getting up to 13 hours per week. If you are just doing SSB LV, you will top out. Winter weight lifting and longer zone 2 rides will help enormously.