Due to having similar problems to you I have had a SPC for four years. I am Certainly not as active as you but , I have been working on a house renovation for the last couple of years and the SPC does not hinder me that much, lots of bending, stretching, crouching,etc.
Wether you will leak in 30 minutes with a flip flo will depend on your bladder capacity. I cannot hold more than twenty ml of urine. Bladder spasms are an issue but mine never seem to happen when active. If I walk the dog bladder drains and no issues, when I stop walking I will leak or get a spasm that causes a leak. A strong pelvic crunch will sometimes stop the leak and force the urine down the Catheter.
Not sure about the scuba diving, I shower rather than taking a bath, I belong to a FB site and there is a mixed opinion about bathing and swimming with the SPC. I will ask a question.
Getting the right fixation device for the Catheter is the number one priority the statlock device is great but it doesn't fit all Catheters. Leg sleeves and straps etc you learn what is best for you as go on. A tap lock for the bag or valve saves wet socks.
I can imagine cycling with a urtheral Catheter would be very comfortable.
I was going to have uretheral reconstruction surgery to sort the stricture but unfortunately the bladder shrinkage meant it was no longer viable. I would also imagine the reconstruction surgery would have an effect on serious cycling.
The SPC is not without its issues, infections can be more frequent, I don't get that many. Blocking of the Catheter is another issue but a quick flush a the right fluid intake helps that one, I have never had to have an emergency change of SPC.
Routine change frequency will vary depending on the build up of sediment in the Catheter.A Catheter should be changed after a maximum of 12/13 weeks. I normally get a change every 6-8 weeks ,if you can last 12 weeks you are doing well. Some people have horrendous experience when having their are catheter changed. I have a routine which makes mine pretty much pain free.
Positioning if the Catheter within the bladder can make a massive difference, I get a slapped wrist from the District nurses for some of the things I do.
My alternative would be bladder removal but the bladder surgeon is concerned that the SRT has damaged the bits they use to form the conduit.
Any more questions, just ask.