Going through those config updates in docker did /something/ as now my local site runs on domain.local without ports, so I presume that fixed the routing issue.
As this was taking too long to sort though, in the meantime I ended up taking the other users advice in this thread and user LocalWP to set up a dev environment (https://localwp.com/)
This not only works, but performs miles and miles faster than DevKinsta has been for the last while (0.5s page loads vs 20-30 seconds) - related to an issue I reported a long time ago but never actually got properly resolved.
Because of these changes though a lot of assets/elements in DevKinsta environment have been purged/migrated to the LocalWP env, so the Kinsta one is in a bit of a sorry state now (my own fault!) - but still loads slowly…
I’m probably going to switch to LocalWP at this point because it works, and it’s miles faster when it comes to performance.
I don’t imagine this is the end of my DevKinsta journey as we host everything with Kinsta, but I feel very unenthused to continue trying to use it for the time being, now knowing that the performance issues are related to the software and not the specs of my laptop, and equally how long this took to get things half resolved. Hopefully at some point in the future these issues can be fixed?