I quit getting haircuts back when my now-2-year-old was born, it's currently down to the middle of my scapulae. I was putting it in a man bun and stuffing it all under a synthetic skull cap, but I'm now to the point that I cannot stuff all of my super curly hair into something so form fitting. If I tie my hair back then put the cap on, it wont sit on my head at all, it just slides up on the top most layer of hair.

Is there anything out there like what Headsweats makes, but with thick, thermal material? I.e., a thermal skull cap that velcros or ties in the back?

Look for one that has a hole to stick the ponytail through,

or cut a hole in the cap you have.

https://www.amazon.com/dp/B01CH1A2L6...il_aax_0?psc=1

Some Pearl Izumi Barrier headwear has a "ponytail port". (I don't have long hair just noticed it on a thermal headband I got as a gift so I can't vouch for its effectiveness)

I've had long hair for years. For bike rides sometimes in a ponytail, sometimes a manbun. I just use bandannas year 'round. Same as my granddad. In fact I inherited a couple dozen of his old bandannas and added some of my own. Larger is better, easier to tie behind the head, over or under the ponytail as preferred.

No problems with the helmet slipping. Warm enough for most of our winter days. Occasionally I'll wear a hoodie. I have a thin long sleeve wicking fabric hoodie for moderately cold rides, a Nike Dri Fit hoodie for really cold rides, and both fit under the helmet. For really cold or wet weather I'll wear the hoods on my Shimano Storm Jacket or Columbia parka over the helmet. And I have ear warmers, those springy things made with microfiber fabric.

Helps to try some helmets to find one that has enough clearance in back for ponytails or buns. Some mountain biking helmets are better, but my Bell Formula MIPS has been fine.

Apparently, i am Canklecat with a slightly healthier neck ( I make up for it with bad shoulders.)

I usually keep my hair in a tight knot on the top of my neck--it is outside the helmet (if ii need to wear one for a gruop ride) and hold the bandana in place nicely otherwise. The knot (Not a "manbun;" I don't even know what they are, but they sound nasty) is small enough (about half a fist) to fit inside most knitted headgear---at least, I have never had a problem. I'd imagine with a skullcap, if you gathered and rubber-banded your hair below the bottom of the skullcap you could fit the skullcap and let the ponytail hang.

You could use the Japanese alternative---samurai used to make a flat ponytail and pull it up over the top of the head and fan it out. might fit in a skullcap that way.

I'd just get a slightly larger skullcap, perhaps ... or cut a hole (and stitch the edges so it doesn't unravel.)