Putting an animal through a lot of veterinary treatment isn't always the kindest thing to do. It's painful and stressful and it's not like they know anything about what's going to happen or have feelings about after they are gone if it comes to that.
Plus when you have multiple pets I agree that self insurance, ie saving the cost of pet insurance and earmarking it for veterinary treatment will almost certainly be financially the best thing to do. I've been in two minds about this with our own brood and we've only really kept up with insurance because two of our cats are on a very cheap 'fixed for life' policy that is no longer available but was an opening offer when Many Pets (previously Bought by Many) started up. Theirs is only £10 a month each. We have another three that cost £10-20 each for various reasons based on age and the fact that one is semi feral and is never going to be co-operative with ongoing visits to the vets, so we just have a basic policy for her in case she survives a road accident or similar so it will pay for an operation or two but won't cover endless scans or life extending cancer treatment - if that happens we will see what she will tolerate and see what the policy covers but it might be time to let her go.
Likewise we have an 18 YO who's no longer insured because it was over £500 a year for her alone and that's with a high excess and 20% co-pay. She has a vet phobia and is very unco-operative and stressed when she goes for her booster so definitely won't let the vet examine her or take ongoing medicine.
OP you could look at shopping around but that isn't always possible if any of your cats have pre-existing conditions, but I think Many Pets do cover these, after a qualifying period of a few weeks or so - might be worth checking them out, especially as they have good multipet discounts. We've always found them very fair with paying out, although not all their policies cover dental treatment, which is something that cats often need.
But on the rest of your costs, you need to carry out a good review of your bills to make sure you're getting the best price and not paying for subscriptions you don't use. Also work out where your disposable income is going - many people who feel like they're not saving as much as they should be able to are often spending more than they realise day to day, so assuming you mostly spend on card, you could download all your transactions for 2025 to categorise and see where your money is really going. Or most bank accounts and credit cards now give you this info within online banking.
This can really be worth doing, because, while you might think it's not worth it for a few quid here and there, cut a few bills and reduce a bit of spending and the savings can really add up, and it's all an increase in the amount of spare money you have to save. Have a look at:
https://www.moneysavingexpert.com/family/money-help/