I sadly have no relevant advice on data recovery other than validating physical connections & trying to see if drives work properly when directly connected to the motherboard. Reason being, even if some cli magic can bring the pool online, the underlying hardware & therefor chance to get data out if questionable.
If you don’t have enough sata connections on your motherboard, but have a second system, see if you can temporarily make a TrueNAS boot (usb is fine, this is just to copy data), move the drives over directly to motherboard, and see if that gives any joy.
As for port multipliers, they might ‘work great’ at first glance, but they generally set you up for failure. Frankly it’d have been better if they just immediatetly never functioned.