I wouldn't be using a spermicide. There are too many side effects for my liking, and they are not strong enough to stop a potential pregnancy unless used with another barrier method. You also need to reapply them regularly, so a long session may not be possible unless you stop and reapply.
Also, I think in the UK you need a prescription for a decent one.
Have you tried putting a bit of lube in the condom before rolling it on yourself? Not too much. You don't want it to slip off. You can also get very, very, very thin ones. We use durex nude. There's no condom scent, and they are super thin.
I'm the one who for various reasons doesn't want additional hormones and hates condoms. Mine won't have the snip either. Which is fair enough.
The only other thing I can suggest but it does come with risk is to track your cycles and use lh strips to pin point ovulation and verify by temping. You'll have safe windows.
Speaking from experience and a baby currently, moving forward we do have unprotected sex but only after I know I have ovulated. Usually around 72hrs after not before because we are truly done having kids.
Depending on age/cycle etc ovulation may not be as regular as you'd like. Hence, my newest baby I ovulate super early, and if I miss my lh surge somehow that month, we always use condoms.
We do this knowing that there is a risk of pregnancy and that it will be dealt with if it happens.
Unfortunately, I don't know much about endometriosis and how that may affect your cycles.
Don't be guilted into doing something you're not comfortable with. Read up, do some research, and speak to your gp they may be able to advise you. Any choices you make should be informed and that you're comfortable with.