i saw that one disability-related post a while ago about dental care but i can't find it again so i'm gonna mention a few things as a (mild to moderately) mentally & physically disabled person, whose teeth hurt when i eat sweet stuff now + i've had a root canal due to a fall + i'm learning to take care of myself, that may or may not help other folks with their dental routine
note: i am not a dentist & this info is mostly what i've gleaned secondhand from dentists, so yeah, i am not an expert in any capacity
water flossers aren't as effective as regular flossing (doing both is actually what's most effective) but they're better than nothing & helpful in cases where coordination problems or other issues might prevent regular flossing technique (i imagine this also depends on the quality of the water flosser)
if you use one of the plastic pre-made floss picks things, rinse the string off in-between each time you use it to floss one side of a tooth, it's tedious but it mimics moving to a new (clean) part of the thread like you do with regular floss
if you get an automatic toothbrush, get one that does circular rotation rather than just vibrating or whatever, as this mimics the tools they use in a dentist's office + imitates the circular motion you're expected to make with a standard toothbrush (which is also hard with coordination issues), i got one that does this pretty cheap from walmart (it's an oral-b but i forgot the specific type) + it automatically times it for you
if you have white spots on your teeth that are uneven with the shade of the rest of the tooth those are potentially white spot lesions due to demineralization; whitening products can make this worse rather than help it, but some products can help with remineralization such as mi paste topical tooth creme, which contains calcium and phosphate (i have yet to try this myself but it seems to get suggested by a lot of dentists, orthodontists, etc. for white spots on teeth & it's also supposed to help with sensitivity and tooth health in general because the white spots are lesions so it's not purely cosmetic!)
it's advised to not rinse your mouth immediately after brushing as this potentially dilutes & reduces the effects of the flouride (if you use fluoride toothpaste), stuff says to wait at least 15 minutes or so
just giving your teeth a quick scrub (even if it's without toothpaste and just water) is better than not brushing your teeth at all
if you have trouble seeing a dentist for financial reasons, try to see if there are income-based or charity dental services in your area, sometimes dental schools also provide low-cost/free dental care
that's all i can think of for now, i wish i had more advice for people who struggle to be able to brush their teeth at all in general but this is all i got unfortunately :(
additionally - you're not bad, useless, gross, or a failure if you struggle to (or can't) maintain oral hygiene; this stuff is much easier for some people than it is to others & those who take it for granted like to forget that, no one deserves to be mocked or looked down on for being disabled & struggling to/not being able to do """basic""" stuff like this!