I self catheterise for a similar reason to you I think, because I can’t make the external sphincter start to go, this means I’m using a catheter most times I use the loo, it’s not just to completely empty, but to start in the first place. This means that I don’t have a residue of urine sitting in the bladder, so (as far as I understand it), I don’t get a UTI because or lack of emptying, it has to be due to the introduction of bacteria.
I think this is the reason why I tend to get infections when I’m on holiday or using toilets away from home so the whole mechanics of catheterising is different. And I have had an infection every time I’ve been on holiday for the last few years. Last time I was away I used d-mannose for about a month, a week before I went, while I was away, and when I came home and I didn’t get a UTI. It may be pure chance, or maybe not? Some people swear by d-mannose and reckon it’s the reason they don’t get infections. I think it’s like a super concentrated sugar which is also found in cranberries. It might be worth a try. The blurb about it says it creates an environment in which bacteria cannot thrive.
I was also wondering whether it might be worth you trying a different brand of catheter; maybe one that is more flexible, or less flexible, or with a different type of handle, so your hand is closer or further from the bit that goes into the urethra.
Or, you could use a catheter more frequently, so there is definitely no residue sitting in your bladder. Do you manage to do it just sitting on the loo or do you have to use a mirror. I used to do it twice a day but gradually it’s become so I do it most times, if you are getting infections because of a residue then emptying completely more often might help? To do it as often as I do, I think you have to be able to do it fairly well just sitting on the loo. Otherwise it would take forever. I did ask my local bowel and bladder nurse and she said it isn’t a problem using a catheter as frequently as I do (maybe 6 times a day).
Failing all of this, you could try the antibiotics, I think I’d feel much like you, not wanting to be on them all the time, but if you try everything else and still get the UTIs, give it a go.
I definitely wouldn’t give up the catheters though. It maybe that you’d get just as many infections because of failing to empty your bladder so you’d be in the same boat.