I only use Pure Pursuit, have not tried Stanley after the first day out. I am not sure with stanely how to tune it.
With PP if you are to slow to a straight line try increasing your Motor low speed, if its too low you will be slow to recover on a straight line, if its too high you will oscillate on a straight line. Most of the straight line driving corrections never exceed the motor low speed.
Motor max speed is mostly for uTurn. If its too slow you will drift away from the u turn and not achieve it, If its too fast it will whip you uncomfortably into the turn.
My settings for PP, think are a good starting point
P 96 multiplier for the steering error entering PID (proportional gain)
Max 196 Speed for fast uturn movement (end of ramp)
Low 44 Speed for normal corrections (start of ramp)
Min 22 All pwm under this speed are ignored (control dead band)
Look ahead 2.4 Point control is driving towards (too high or too low oscillates pick the middle)
Look ahead gain 1.8 (Look ahead speed multiplier, seems to help)
deg/deg 0.5 (automatic steering degrees uphill per current side hill angle)
Integral 2 start at 1 and work up in very small increments, Increases the aggressiveness of the steering correction the longer time the error exists. handy for slippage.