My ISP's security strikes me as... somewhat alarming to my admittedly non-expert eyes. They store wifi passwords in plaintext on their website and allow changing it there - this is apparently meant to be the only way to change password/network name, too. They provided a router which I assume they want to be administering, as they didn't give me the password... except it was something I could guess in a minute. Their response to my phone starting to bug me about security settings (after a recent wobble where the router supposedly reset to the saved-in-plaintext-on-their-website settings but I couldn't access it with these, to boot) was a plain "eh, ignore it" without explanation, and a claim that I had "the latest security (WPA2)".
However, they're also the only ISP in the area.
What can I mitigate, security-wise, and how, broadly? (Or am I just being paranoid?)
Specifically for the router I've seen an answer about using the ISP-provided router as a bridge to a router of one's own, but I'm not sure how much that will actually remedy - e.g. in terms of being able to use WPA3 on the non-ISP router,... I know there's been a question about that as well, but the details seem different.
I also assume the task of securing the network is not done with just plugging my router into the ISP's (and subsequently leaning back and ignoring the ISP and their issues exist) - what else can/should I do so I don't end up leaving something else wide open?