Hold Inventory or produce to order?

Hello all, very glad to have found these forums. I run an online custom novelty t-shirt business that has over 400 designs and has done quite well over the years. What I am starting to question however is my process and wether or not to carry a staff. I have always produced all my t-shirts to order, meaning when a customer places an order on my website myself or one of my employees screen prints the design the customer chooses on the size and color the customer chooses. Anyone that is in the screen printing business may know this is kind of a crazy idea when considering setup time BUT 95% of my designs are 1 color and are super easy to setup and I carry very little to no overhead operating this way. Staffing can be a challenge with work flow somewhat of a roller coaster being that we are very busy around the major holidays and slow down quite a bit the rest of the year. I have a full time manager and a full time printer but I bring on part time employees throughout the year when we need extra help but training someone to only work for a month or two is tough. And when you do bring basically seasonal part timers they don't seem to be as serious about whats going out the door as I am.

I have been doing more and more thinking and I honestly believe If I could just narrow down my t-shirt designs to say my top selling 100 and just print them myself or even outsource it I can run this business entirely by myself again. Especially when customers receive a badly printed shirt or the wrong shirt design entirely or I look around and employees are just sitting around looking for something to do when its slower. Only problem Is if i went back to the route of me doing everything and stocking designs I would be sitting on thousands of dollars worth of Inventory Instead of printing to order and keeping my overhead low.

Thank you In advance for any feedback and or advice.