My kids are 6 and 4. Both of them have gorgeous tight-coily-textured hair and both of them have (different) sensory processing sensitivities.
For the first time in our family’s entire life I successfully washed and cared for everybody’s hair thoroughly in one weekend — even less than that, it only took a few hours!
Using an inflatable tub on the floor of the living room while watching a preferred TV show was a massive game changer for us. It’s also letting us split apart the concepts of “wash your body” and “wash your hair”, which is helping us further problem solve down into the granular specifics of how to keep everybody clean, who is willing to take a bath or a shower and why and when, etc — all kinds of collaborative problem-solving goodness* that we can do now that both kids have the capacity to have verbal conversations at a higher level than they could even just a year ago…even really just a half a year ago.
For years, we have tried out every idea that I could think of and gone through quite a repertoire of things that worked for a little while but not beyond that, including both extremes of that spectrum of “just try to power through it despite the tears” and also “just do nothing at all” and also “just cut all the hair off so we don’t have to deal with it”. And I have no doubt that we will have to try more things in the future.
This is why I am very infrequently dogmatic about there being “one right way” to do things. There may be multiple appropriate ways to do things right this second with the kid right in front of you, let alone if you start factoring in how the kid changes over the years, the parents change over the years, the resources and environment changes, the language processing ability changes, the exactitudes of sensory processing difficulties change…
Keep doing the best you can do. Keep believing your child when they tell you (with words or behavior) how something feels for them. Keep reading, keep paying attention, keep listening, keep learning, keep trial-and-erroring new things.
Also if you need an inflatable bathtub I’ll put the link to the exact one I used in the comments but all I did was search for “inflatable hair basin” on whatever website you want to buy from. 🙂
*For more on collaborative problem-solving as an actual step-by-step strategy laid out for parents to work with their kids, look at any of Dr. Ross Greene’s work, such as his book Raising Human Beings or his work at Lives in the Balance.