People will always happily wait for quality.

If they aren't happy to wait, then they have a choice to do it themselves, which ofcourse, most can't.
In this crazy world of technology, people have lost the concept of patience.

You need to seriously look at putting on an extra staff person. Pay them a rate that works for both of you, teach them and the business will grow and get through the work.

I saw Kenny Priest run his pinball business up here and I watched his high end pinball restorations.
He used extra passionate staff when he was swamped and I watched the business closely for many years.
He paid an amount on a ABN basis and his contractors got paid a fixed price per machine. He oversaw a work, managed it and checked quality control.
Without getting extra staff, he would never have been able to keep his backlog down from 100 to 10 machines.

Getting extra staff at a lower fixed rate gives passionate pinball people a job they love and the smaller earnings are the bonus. You would be amazed how many people would be willing to get involved and a lot will already have skills.

The company who purchased Kennys business up here when he retired copied his model and it works very well.
Everyone is making money and the business is growing in size and turnover, not earning less. Food for thought.