Maybe start with a Dramamine? ;-)
I feel your pain. I tend to research things to death, myself. Just
one more Google ...
Now that you've got too much data, try making a list of Pros and Cons for the various options/decisions. And once you THINK you've made a decision, don't do anything until you've slept on it. Our subconscious literally works overtime for us, if we give it a chance.
^ The advice above is solid.
What follows below is mere opinion and the relating of what I recently did myself.
Transfers can look nice and have a soft hand, IF they are for white fabric. Dark fabrics require a thicker, heavier transfer that does not wear as well ... but such quilts are likely seldom washed compared to a T-shirt, so perhaps not an issue for your use? Still, if it is important to have a soft, non-plastic feel, then transfers for dark fabric are not really an option. JPSS is great on white fabrics, soft hand after one wash, and durable.
Printer for quilt transfers: I just bought an Epson WF7210 for printing JPSS transfers and some photos/art prints. They have been on sale recently for $149 with free shipping. It is a 4-cart printer CMYK, which most agree is plenty of capability for printing transfers for fabric. In my opinion, a P400 is overkill for transfers, and will just cost you more for ink in the long run and more for a printer up front ... and you will cry when the head clogs, rather than shrugging and buying another $149 printer
It can print 13" wide, and can hold up to 13 x 19 media in it's two paper trays. Also, I find that it prints photo prints quite decently well, while being less bother than my 7-cart Epson SP2200. The OEM Epson ink will shift color a bit when heat pressed, so I bought refillable carts and pigment ink from Cobra. They provide color profiles for use with JPSS as well other popular papers.
With all the money you save by not buying the P400, buy another WF7210 and equip it with carts and dye sublimation ink from Cobra. You don't need a printer that large to print mug transfers ... but who knows what else you might want to sublimate on some day ... and besides, printers only get so inexpensive, no matter how small they are.
Mugs: Something I have been considering myself, as well as the other doodads and whatchits you can sublimate to. I noticed that there are at least two approaches to sublimating mugs: A special heat press for mugs, or special wraps that hold the paper in place and bake in a convection oven. So one is a piece-by-piece process, while the other is a small batch process. I don't have an opinion on this (yet), but recognize it as one of those things that needs some data collection and a Pro/Con list or two. Both ways work, but which would work best with the way you want/need to work for your business model?
Packages: I'm not a fan. As you've noted, they inevitably contain stuff that you know you do not want. Best to source each thing to best meet your needs.