been in a very similar place for many years, much better now but I still find hygiene hard, but it's a lot less to do with my body now. setting specific days /time / having a routine for it even if it's not often as ideal helped me, I find routine hard to maintain but it did do something. if you can consistently shower once a week, do that. setting the bar at ideal/perfect circumstances is what gets us down because we aren't there yet.
also I used to shower exclusively in the dark.
and lumping in other needs with showering ie. combing hair or something like that. so when the less distressing one needs tending to it kind of forces the other to be seen to too, while you're there.
I have long hair, my showering routine kind of revolves around looking after that since that's the thing that will force me in there when physical hygiene wont.
having to be somewhere can be a good motivator for me to shower the day before, deadline of sorts.
it does get better though, recently I've found myself wanting to shower more, and more readily than probably my entire life, and it's mainly other barriers than having a body, nowadays, that make it more infrequent. before I was borderline phobic of being clean and I didn't realise how bad it was until it started getting better. multilayered trauma thing. but it is common to struggle with over/under cleaning post-trauma.
other things I used to do to remedy maintenance related stuff was to try and make things as easy as possible that I can control outside of the showering itself; struggling with being naked? new clothes in the bathroom so you can get dressed asap. hair neglect? why not have it shorter so it needs less frequent care and is over and done with quicker/easier when you do need to wash it. cold bathroom making it feel even more challenging/offputting? space heater 5mins before showering. feeling gross but can't shower? change/wash clothes/pyjamas/bedding.
for me it has to be very easy / as little moving elements as possible for me to have a good chance of following through alright. so not a day where I have something on, or a parcel coming, or other time sensitive/stressful things. making sure the shower is relatively clean/working properly also helps a lot. which is stuff I struggle to keep up with or get help for so is an obstacle.
edit: to add. if you have friends/family who understand or you feel comfortable with, just messaging/saying that that is what you're going to do today (eg in response to "what are you up to today" or similar) can help lock in the task as something you will actually do even if it is very reluctantly. that has helped me when in tough times with it. being able to have a "how are you?" "I'm okay, going to take a shower tody" exchange. can be encouraging if you know someone who has similar(ish) issues with it.