I do exactly what you do, as does PorkChopHarry. But one difference--at most we do 3 colors (only one active design for me), maybe eight 2 color designs for me, the rest are one color. The pain increases exponentially with each color!
Although that 3 color was my best seller back during the primaries, generally the single color designs sell better than multi-color, at least that has been true for me. But Horror may require torrents of green and red in addition to white ...
Are you printing waterbase inks? I am. So any sort of Plastisol, a transfer or not, probably feels like poop to you ;-)
I set my turnaround times so in theory I would never have to print more than twice a week, and sometimes can get by with just once. That increases the odds of being able to print multiples of a design ... still, one ends up printing singles much of the time. The time spent on cleanup gets old.
In his early days, PorkChopHarry tried to print ahead, but it seldom worked out. If this week you print a few extra of a design, then next week it won't sell at all, or it will sell 6 units in a size that you only printed 3 of.
I have messed around with JPSS transfers a bit in the past, and recently decided to scale that up. It is not a replacement for the typical stuff I screen print, but rather an outlet for more colorful designs, of which I could have thousands and POD myself without the chasing-my-tail dizzies of POD screen printing. But there are tradeoffs. JPSS is really only for white garments. Especially for Horror, that probably wouldn't fly. Unfortunately, as far as I have been able to determine, there is not a transfer for dark garments that lasts and has decent hand feel and is not too finicky to deal with ... also, on the laser end of the options the materials and printers are a bit $$$$.
I have a Merch By Amazon account, so have seen some of my designs done with DTG. The quality was better than I expected, but I hear it can vary from machine to machine. To invest in the setup to do that myself ... I don't know. For $220 I have an Epson and Cobra ink with which to print JPSS. If the head clogs beyond recovery, I can buy another and not cry about it. But $15,000, or so, for a good DTG ... that may or may not die of White Ink Obstruction Disease if not constantly used ... That is a lot of capital to invest in something that can become a useless brick (as opposed to something like a Vastex screen press that would cost a third as much and still have resale value after years, and years of use).
Have you tried wholesaling to shops/stores? At least then one gets to print a batch of each thing, even if making much less per item. I got all setup to do this, but have not pushed on the marketing end of it yet ... marketing type things in general not being very appealing to me ... undoubtedly my greatest failing as a "business person."