I only ever gave mine the first one and one booster. They are now 10 and so far there’s been no issues. My late girl was just under 15, she also had vaccinations only until she was 5, with no repercussions.
I do volunteer at a cat rescue so know I could be bringing bugs home, but I change as soon as I get in. Mine have a catio and supervised garden access.
I decided not to vaccinate when I realised yearly jabs are not the norm in all countries. Plus in some other countries they jab in the leg, in case the site goes cancerous they can take the leg off, much more difficult to deal with in the neck.
It won’t invalidate your insurance, only means they won’t pay out for illnesses related to not having the jab. My boy fat boy got sick last year, probably from licking a frog and insurance covered him
We don’t jab the ferals where I volunteer (not easy to catch so many free roaming, mostly human shy cats) and they don’t get sicker than the domestics.
Basically do your research and make up your own mind. Be aware if you put them in a cattery, they will require them to be vaccinated though.