Summary

I couldn't add models to my make so I had to remix to add them. Copy and paste from it:
CarriageSrpingV1 - I ended up making a Carriage Spring because there was too much slop. When a direction switch was made the pen would stay still until the slop was gone and not make letters. It just slips around the normal carriage with some springs between the two to give it tension to the rail. The cross-bar that the servo actuates could have done this also but my stock was a different size.

PlotterRoller - Just a model for the rollers in the back. Plan is to try to sleeve them with some material and then spray with PlastiDip.

ServoTryTwo - This is a mount to double-sided tape into the solenoid spot to fit a standard servo and orient it to not need a linkage to use the same setup you have. Just orient the arm after you'v homed the servo so that it's not pressing against it then.

Shaftbuddy - I would suggest searching for the same model here. It's just a coupler from the stepper shaft to the ~13mm pipe stock I found.

I used a version of GRBL modified for Servo Support to add the M03 commands. M05 also homes the servo. It's hooked to pin 11 via Z+ endstop.
M03 S10 picks up pen -- Laser disable
M03 S0 Puts back down -- Laser Enable?

I used two 3/8X3/4 spring in the carriage spring. Can fit others.

I'm not sure what the limit switch on this model is for as it does not seem to be hooked up in any real way. I set it to enable halt so the carriage doesn't crash home, but would like to see about getting it to be a soft-home instead. Plan for completion is to plasti-dip both rollers and the 13mm pipe as well.