A lot of the options that come to mind are in the 20K-undergrads range. Is this definitely too big? A lot of these schools don’t feel huge.
UDel was already suggested.
James Madison was the first that came to my mind - walkable campus/town, nice undergrad experience, lots of options both academically and extracurricular-wise, and a great-yet-accessible marching band. (There are levels of participation; no audition for wind players to do the pregame show, but there are auditions for the smaller subset that does the halftime show and travels to away games: Marching Royal Dukes - JMU )
WVU could be another good option, but does require an audition: About | WVU Bands | West Virginia University
Towson, in Maryland, is another. TUMB Site – The World Famous Towson University Marching Band
SUNY Albany is in range too, and matches PSU in-state tuition for PA students. Has a nice honors college if she qualifies. UAlbany Marching & Spirit Bands | University at Albany (Maybe too urban but seemed worth mentioning.)
Rowan in NJ could be worth a look too: Bands
If you stretch your driving radius by another 1-2 hrs, you could also look at Appalachian State and Miami of Ohio.
It’s harder to find robust marching band programs at smaller schools, but it looks like you’ve already identified a few.
Guessing you’ve already looked at West Chester? Incomparable Golden Rams Marching Band - WCU of PA It seems to fit the parameters quite well, and is a little smaller than the others I listed.