I think your problem may be the connectors on the Solenoid expander board under the playfield. Reflow/replace the headers there & check the relay terminals for cracks/dry joins & check the female connectors too. In lamp test mode check the dedicated lamp associated with the SEB is present & flashing with the rest of the lamps. The relay should be clicking as well.

Failing that, check/replace the female connectors on the solenoid driver board J4 if they have not been replaced (good to replace anyway) as you have tried several boards so probably not male connectors. These are the solenoid data lines that come from the MPU (J4) most likely not the issue though as a J4 connector problem would usually cause erratic or wrong coils firing rather than a weak coil(s)

Check the manual, I believe there is a switch setting for that.

Repinning the MPU connectors won't help. They're just sending binary data to the driver board, a weak connection won't affect the strength of the pulse. The MPU's coil pulse times are controlled by the zero crossing signal so it's not really possible for them to be off either. Manually shorting the transistors is not a good test since you can't control your timings. Repin the connector for the coil on the driver board, check connectors on the rectifier board, clean/replace the playfield coil fuse holder, etc

Switch test mode operates the coils for a slightly longer period than game mode so will appear stronger.
You also want to make sure it's not a slightly binding mechanical issue with the mech.

Odd one. Just for fun, test to see what DC voltage you are getting at the coils for the targets. Have you replaced the Rectifier board? (or at Minimum the fuse clips). Have any rusty, 40 year old fuses in it?