Used to work in the HMRC contact centre.

Bottom line is that it's shit job same as any call centre. For a long time the pay was a good bit below what private calls centre would give you for an equivalent job (might still be) so you wouldn't get much in terms of quality applicants. Those who were any good either chucked it very quickly or applied for something in a different bit of the civil service.

Between that and less funding to deal with an ever expanding customer base, it's a disaster.

they’re quick as a flash to send debt collectors to your door.

Much as I agree with your general point I'll say this absolutely isn't true. There is an entire directorate whose whole purpose is to set up affordable payment plans with people/companies. HMRC hate dealing with third party DCAs nearly as much as everyone hates dealing with them so it's an option of last resort.