One year update: I went back (again) to dark red henna last fall, after lightening to copper last spring/summer. I ultimately decided, sometime in October, to grow out my natural hair color, which is lighter and ashier than my henna. I still absolutely love copper & red hair, but I've come to realize that my natural color suits me best.
I'm really happy with how the grow-out is going, so I thought I'd share what I did for any others who are growing out a slightly lighter color. I don't think this would work on blondes, but I have light golden brown hair so this might be useful info to anybody else in the light brown camp.
Basically, once I decided I was going to grow out the henna, I let a decent amount of roots grow out so that I could gauge the color. Then I took pictures in different lighting to reference: https://imgur.com/a/7BjVidW (parted my hair in a weird spot so I could see the virgin color without sun exposure). Then I did one last root touch-up so that most of the hair was the same color: https://imgur.com/a/ViZXUYP.
I waited about a week for the color to completely oxidize - not too long that any roots would grow since I didn't want hot-roots - and then I did several rounds of Sun-In all at once, with high heat. I soaked in coconut oil before and did an Olaplex treatment after. Very little damage to speak of, I still haven't noticed any and I also blow dry multiple times a week on high heat (currently I'm maintaining at collarbone while I grow out the henna).
It's now been 3 full months, so I started taking pictures of the new growth compared to the henna. It's obvious to me, but not many people noticed the roots. Just one person could tell. The Sun-In treatments seem to have a browning effect at the same time as the lightening, at least for me. Under fluorescent and natural lighting, you can see the difference pretty well, but not in most indoor lighting.
Here's what it looks like indoors: https://imgur.com/a/iQYkHs8
Here is the most obvious I could manage to get the roots to look (direct sunlight and fluorescent): https://imgur.com/a/2TjPszS
So... yeah. All things considered, I feel like it hasn't been that painful. I would really recommend doing this Sun-In thing for anybody who is wanting to grow out their slightly lighter roots with henna. I also tried dying my whole head dark brown and then black, which was super awful looking, but that would probably be another good alternative because it did completely wash out (I used Ion semi-permanent in Black and Clairol Beautiful in Dark Ash Brown... or something like that, I can't remember now).
I also plan on visiting a colorist sometime in the near future to do one last mini-lightening session and a tone. I'm guessing I will have to continue to tone the bleached hennaed part until it grows out, but I'm not sure how that works. I can't have obvious grown-out roots for my job, so I may also employ some headbands for the next several months.
Anyways, that's pretty much it... I hope this is helpful to somebody. I was very nervous that this was going to suck way more than it is, so I'm pretty relieved so far.
