How do you provide more workers to your city?

Step 1: Look at unsold goods. Bulldoze commercial of appropriate wealth levels until unsold goods are close to 0. Now those commercials will be taking in the right number of workers for the goods required. I expect this will free up ~2000 workers in your case.

Step 2: Bulldoze high density industrial... constrain it to medium density. You don't need super dense empty factories. I expect this will free up 3000-5000 workers in your case.

Step 3: Service buildings. Consolidate police and fire into fewer larger buildings. Each building requires workers while the extensions do not require any more workers. Check the number of workers at a service building by pausing the game, checking the city worker count, powering off the building, comparing the new worker count (you can power on the building at this point if you want to keep it). In particular, things like CoalMines and other specialization buildings require a lot of workers. Limit the number of these that you have.

Step 4: Residential density. If it's not all high density yet, you have room to grow. If you're having trouble balancing the traffic of low wealth residential, make a community college or university to upgrade your industrial and focus on medium wealth residential.

Zone (or rezone) more residential areas to further increase your population. Increasing zone density in your existing zones will increase the number of sims living in your current residential zones without having to add additional zones, but will require a source of happiness.

A good practice for rezoning that I follow is that as buildings of a particular type (in your case industry) are abandoned, I generally rezone them as residential or commercial.

If you don't have anymore space in your city to zone more residential zones, you could either look at rebuilding part of your city to provide additional space for residential zones or you could create an additional city in your region (or get a friend to do it for you) with the purpose of housing a large number of residents that will become additional workers in your city.

To resolve the issue of undelivered goods, you could build a trading depot, which will act as a buffer for the goods and allow you to distribute them out of your region.