I'd probably need access to the credit if needed tbh;- I have 3 cards with the best APR still being below 20% (19% or so I think) with HSBC and other actual banks (none of the dodgy ones).
My credit rating fluctuates between Excellent, Good and Fair (a few points off Good) depending on whatever is happening, but if you do make those payments it will continue to build or maintain your credit rating. Having all of the cards maxed out can hurt you though - say you're using somewhere between 30 to 45% for you limit continuously or maxing it out at some stage. If they're all maxed (although I understand you have a mortgage that's getting paid off as well) then that might not be great.
Your current account balances will also appear healthy so maybe you have nothing to worry about, just game the system.
Just don't repeatedly apply for cards (and don't apply to cards supplied by the same parent bank, they will reject you if you're already a client - I did this once - M&S bank is HBSC
). And stay at the same address and on the electoral register if you can. Some utilities (including a mobile contract) can also help but I went SIM only forever ago and try to avoid utilities that use credit reporting for strawberry float all reason.
CCs have been helpful for me as business loans and overheads/cash-flow vehicles;- it's basically impossible to get a loan as a new business 9 years ago and even the ones that are OK have terrible interests rates. But for whatever reason at the time I had a good credit rating from being granted an emergency overdraft when I first put a deposit down on a home (renting) myself, and never borrowed before that (having a few thousand in my account and then spending through that, which was actually student finance anyway;- then my only and oldest bank account was more likely to approve me for 0% (I think it was 24months). I took me strawberry floating forever to pay off the overdraft "on demand" though (probably because of the CC balance which was only about 2k;- this was dumb because Nationwide gave me both an overdraft and a credit card, so they basically wanted me to use the credit card rather than an interest free overdraft once that deal was up). Yeah I shouldn't have been in my overdraft and I don't have one anymore. Rent is insane here and we were strawberry floated for bills as well.
Each balance transfer I've kept the card open at £0 or used it for short term borrowing on the lowest APR and I haven't missed a payment in 8 years - had some problems and forgetfulness (I think I bought ice cream or some gooseberry fool and empted my current account in a pay lull which meant a payment couldn't go out) so my first CC got nuked from orbit but that's it I think. I'm not 100% sure but having a card with a high limit on it that you're not using is an indicator you don't just buy a gooseberry fool load of things you don't need because the credit's available. But there's a theoretical limit on how much total limit you can have, based on your means. So if you needed another % card or just wanted a new CC for whatever reason you might have to close or lower the limit on one card to free up "credit worthiness" for another.
I know for a period between 2008 and 2012 or so (I studied for 4 years during the financial crisis, great timing...) basically nobody would touch credit cards (including me) so after that they were practically throwing them at you. It would cost the equivalent of under £200 to borrow thousands of pounds over 2 or 3 years which was better than any other kind of loan for me;- you just had to not strawberry float up managing them, which is kind of hard now with direct debits generally being set up automatically, and you can of course pay chunks off whenever you want (which I do).
Unfortunately it's the nature of running a business out of a recession (and then Covid) that you generally need a credit facility of some kind but if I was ever cash rich I'd maybe do it. Now there's a cost of living crisis I think the only people buying anything are businesses or business owners funnily enough (probably on credit or loans...).
With no PAYE or chartered or company accounts, it's pretty much the only way for me to build towards a mortgage too, or whatever is next for my business. I also wanted some financial independence even if I meant I had to be straddled with a small amount of debt;- no private loans or unwanted "business partners" (financiers meddling in things, the whole strawberry floating point was to take control of my own life).