In terms of not having a sixth form, that really depends on the local context. In some areas with large sixth form colleges you'll find a lot of smaller schools can't justify a sixth form financially, so that doesn't bother me.
GCSE results it depends how bad. What is progress 8 like? What are results like for different bands of attainers?
High levels of pupil premium is not an issue to me, and can be a good thing as it attracts extra funding for the school.
High levels of SEN - depends on the context. If students with SEN are well supported, the school can easily recruit TAs, there is a good SENco in place, that's good. But high levels of SEN and poor support is not so good.
Needing to actively recruit staff at this point in the school year is a red flag. It means they either couldn't recruit in the summer or lots of staff left this term. Neither is a good sign. This, for me, is the biggest concern about everything you have written. Are you happy for her to be taught eg maths, science, mfl by a series of non specialist supply teachers? Not just for this school year but potentially until she leaves?
What does the most recent Ofsted monitoring report say? There will be something more recent than 2019. Not a full report but similar to a letter. Reading between the lines you will be able to tell if the school is improving or not.
I'm pretty sure that phrasing is standard in terms of appeal. People on here will tell you to give the school a chance but honestly a school with insecure staffing won't give your child the best chance, and if it were me I wouldn't just sigh and suck it up.
If your daughter is good at art then look at the other schools, do they offer subjects like graphics or photography at GCSE? Does one have excellent facilities or better extra curricular opportunities? That can all be grounds for appeal. There are lots of reasons to appeal, if you start a new thread if secondary education you'll get some good advice.