It’s very difficult to overfill a 5HP24. Did you definitely have the engine running while the level was being checked?
If you find red fluid in a 5HP24 you’ve got a big problem. New ZF Lifeguard Fluid 5/Mobil ATF LT71141/Pentosin ATF1 is straw/honey coloured.
Does your transmission operate fine in reverse (just to confirm that the F brake is functioning correctly)?
An occasional issue on the 5HP24 is a crack in the upper valve block in the SV-1 to KV-F circuit. This causes leakage which drops the hydraulic pressure allowing the A-clutch to disengage when the fluid is hot. A typical symptom is that it feels like neutral has been selected when you’re in 1st gear but the gear then engages sharply when the engine speed is raised (i.e. as the pump speed increases the flow exceeds the leakage allowing pressure to build in the clutch). It may be worth dropping the valve body assembly, just to check for a crack in the upper valve block?
Phil