If Apple had your best interests in mind they would provide a way to integrate third party payment systems into their management interface so all apps could expose their subscriptions to the user in a consistent way regardless of payment backend. Instead they will keep it as an exclusive feature and point to other processors' lack of compatibility as a harm to end-users inflicted by the DMA. They would rather have a talking point in their ongoing temper tantrum than provide a good experience for their users.

I actively don't want third-party payment systems on my phone. I also actively don't want the ability to load and run arbitrary third-party applications on my phone. Adding these things would make my user experience worse, not better.

My phone isn't a general-purpose computer in the way that my laptop is. My phone is an appliance. When I use my phone, I'm not actually using the phone itself, I'm using the email app to manage my emails, or the web browser to look at some news website, or Instagram to watch some reels, or Slack to communicate with my colleagues, or etc. The phone is just a conduit for apps/workflows.

You say that, but the Google Play store doesn't allow you to do that either, so do they not have my best interests in mind?

Yes, of course they don't.