First gear uses the largest band in the transmission as you need a lot of torque to get going. Sounds as if the piston that applies this band isn't working properly.
The piston has a "teflon" seal & there are various size adjustment pins to change the band clearance. FIND a local shop that will try to replace this seal & adjust the band(in car).
Should be able to do this in 2.5hrs & $25 or less in parts.

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