Probably will go with the new amp board buying. There are three option for an amp board with bluetooth and volume and tone controls found in Aliexpress:
---This with TPA3116D2:
ZK-1002T AUX+Bluetooth 5.0 Amplifier Board HIFI 2.0 Channel 100W+100W Audio Module Adjustment Stereo Subwoofer Speaker Module
Runs with 12-24V/5A
---Also with TPA3116D2 there is:
XY-T50L 2x50W Bluetooth 5.0 Subwoofer HIFI 2.0 Speaker Digital Audio Power Amplifier Board TPA3116D2 Stereo XY-T50L Home Theater Audio
This run with 5-24V/5A.
---And also this one, which doesn't specifies any chipset:
ZK-502MT Channel Bluetooth 5.0 CS8673E 2.0 Subwoofer Amplifier Board 50W+50W Power Audio Stereo Amplifier Board Bass AMP AUX
And this runs with 12-24V/"above" 3A
Those are the options. I don't know if there are any differences between one and the other, but before buying anything I prefer to ask to you.
Also I see my PU can't work here. I have to find something with more Amps, it seems.