I have a hard time finding a good heavier duty servo that does not cost the price of a car. I looked into how to make one from a DC motor. There are some open source project about it:

https://hackaday.io/project/9433-brushed-dc-servo-drive

https://www.crowdsupply.com/citrus-cnc/tarocco

But for some reason, most projects I found do not use DC motors:

https://www.kickstarter.com/projects/tropicallabs/mechaduino...

https://odriverobotics.com/

If you ever find a good OSS servo, could you ping me?