Next time do the full series of hydraulic pressure tests before taking a trans apart.

Now that it’s apart, pull the front servo apart and check all the seals. The front servo gets line pressure from the valve body - same pressure and feed as front clutch apply - but only on the 2-3 shift and while in third.

You state you have a good solid reverse. If so, then the front clutch drum has clutch capacity.
Even though the front clutch drum runs at higher pressure in reverse, if there was an issue with it’s internal lip seals or the two metal seals between it and the reaction shaft support you would likely have some slippage in reverse.