Booster for large 6 year old

I need advice on a booster seat. I have a large six-year-old. She just turned six in December. She is very tall, probably 4 foot tall. She weighs 102 pounds. I know she is overweight and it is something we deal with. I had her in a Britax Frontier up until about two months ago, using the booster function.
She would complain that it was hurting her legs. Her legs (knees) would extend way past the seat and this caused pain.
I moved her to a safety 1st backless belt positioning booster. It was the biggest one I could find that didn’t hurt her legs. However, the booster does not boost her up enough for the seatbelt to hit her in the right place. It’s too short!
Does anyone know of an alternative that I can try? Maybe even a special-needs seat that isn’t too overly expensive?
I even thought about putting towels under it but not sure if that is safe.
Does anyone have any suggestions?
I greatly appreciate any feedback that you can give me.
Shannon

Does the booster have a strap to position the seatbelt over the child's shoulder? I have a different brand and the strap is helpful.

There's a picture of one in the middle of this page:
https://csftl.org/proper-booster-fit/

Would a lightweight stool for her feet to rest on help the Frontier? It seems all my kids need that at some point because their legs hurt. I use the small one from Ikea.

Have you tried the Evenflo AMP? My 9 year old at just under 5’ and in the neighborhood of 100#s is still using ours.

she might be a good candidate for a mifold - it mostly positions the seatbelt.

You know what, it does but for some reason it doesn’t really move the strap much. Maybe I’m not doing it correctly... Hum.

Clearly I don’t know how to reply to comments... lol.
Thank you ladies, I will check those out right now!!