That's about a $300 fork, and it's not bad....Solo Air right?
Do you have the shock pump, and/or tried adjusting the rebound damper?
100mm travel is typical for a trail or crosscountry bike.
IMO it's not worth spending the extra $ for a new fork, and buyer beware when purchasing used forks.
Keep riding it and save for another bike.
Maybe a slightly larger and cushier front tire would help.