It does sound like you had a particularly bad experience, possibly not helped by 3 closing their UK and india call centres in the early stages of the pandemic. It's also unfortunate that as a society we seem to have sleepwalked into using mobile number as a security token when the infrastructure behind supporting them was never designed to make this as secure as one would want for that purpose.
I guess we all have our horror stories of customer service, my most recent one being the kitchen appliance supplier who offers a three year guarantee, but the first step is an engineer inspection of the problem, and they have no capability to inspect microwaves themselves in the UK, and nobody on contract to do it for them - we are about two months into an escalation at the moment.
I wonder how much better the other mobile suppliers are though, once you are unlucky enough to have a problem. I know from experience at work that higher price doesn't always mean more is being spent on service, sometimes it just means higher profit for someone, or more wasteful incompetence. For what it's worth the Ofcom 2019 figures are: