I know that for my gymnast, L8 and high school honors classes without adding a HS sport made for an extremely challenging year. Adding another sport, and all HS sports involve a significant commitment/demand of some sort...also increases the risk of injuries for many reasons. With the amount of money and commitment that optional gymnastics requires, risking sitting out because of injury from another sport or the combo of two sports is definitely something to consider even if your kid is not accident prone like mine.

My kid is very bright, disciplined, all that good stuff but no way could she ever have pulled off L8, a challenging course load, and a HS sport.
This. The jump to level 8 is beyond 'real,' and I can't even imagine dealing with it while doing high school, let alone with adding something else extracurricular!!! I feel like there are all these "stopping" points for gym. **Note these are GENERALIZATIONS** people, lol!
Those who don't get the bloody kip sometimes head to xcel or retire...so level 4 seems to be a stopping point
Those who develop fear issues of the heinous level 5 (that level will always be thought of as heinous imho) retire before optionals
Those who can't get their Giants stop at either level 6 or 7, depending on if their gym will work with them and allow bars to be competed without them.
Those who have backwards issues also often stop at levels 6 or 7, again depending on if their gym is flexible or not.
To get to level 8 is an incredible, INCREDIBLE accomplishment. I truly feel from reading all these threads over the past few years that it is the gateway level to get you to level 10. I know from all the gyms in our area that many girls who have repeated 7 either head to xcel the next season (which I think is great! It is a way to stay in the sport while pursuing other passions) or they retire. Because the jump is really a jump.
...and let's not forget Summer. A high percentage of gymnasts quit in summer, which I understand. I don't remember the percentage, but it was mentioned on here somewhere.
So, my two cents is that if she does her time in Summer, and pulls off all those level 8 skills in Summer, I would say that gym would need to be first and foremost. I am not trying to sound harsh, because I do get it is an emotional issue, but I really think the decision should be made before summer. That way, if she is going to do other sports, she can enjoy her summer, and begin networking, so to speak. Because after putting in the time, and in a sense, "giving up" her summer (I have heard other middle and highschool girls say this comment), it will be much harder to justify cutting the cord.
Two huge red flags that stand out to me that I don't think anyone else has touched on (forgive me, I am skimming here) are
A. no girls her age at her level, and
B. not the greatest relationship with the hc
You do not mention why the relationship isn't the greatest, but if your child is already in 9th grade, odds are good she has gone through the horrific puberty emotional tween years when relationships are rough with everyone including themselves. So, if the relationship has been rough for awhile, she has already repeated level 7 once, and she is excited about taking these other sports up a notch.....might I gently suggest she is giving you her answer? Level 8 isn't a level to touch lightly on....is there any chance at all for an xcel program, even if it meant another gym nearby? There are some amazingly high level xcel girls out there! Especially as she has already dabbed in other sports, and it hasn't killed her interest....well...I don't see you being able to take those away without some real issues. Also, what are her goals with the sport? That is also important. I know that at level 10, often there aren't many teammates in general, but at level 7 there are usually some a girl can count on that are around her age group. I am sure she is thrilled to work with others at her age group.
Good luck. It is honestly so much harder on the parents when their kids stop doing an activity they have done for a long time, regardless of what it is. I wish you and her the best, and am sure you two will have talks in the future about the future.