This is a very interesting situation.
Almost most of my work is on fabric, usually existing garments, flags, micro fibre cleaning cloths, scarves - that sort of thing.
Since I have a virtually 100% success rate, which means, no loss of clarity, no ghosting, no transfer lines, no spray blobs or any defects of any sort and don't do any of the fiddling about suggested here, I suggest with all due respect that your attempted failed solutions are all experiences which you will come to realize taught you what not to do.
There was a time that fabric sublimation was quite the challenge for me as well. Read extremely high failure rate. However, I took a logical approach and tried a bunch of things, documenting the results. Where I saw improvement, that process was built upon.
What you could really use to speed things up in developing your skill is someone in your area who does fabric well and is willing to show you how by doing an actual demo at your place with your equipment. There are lots of variables which are fine points to ensure your work is great and not just OK.
Once you develop the skill, it becomes mechanical. Just repeat as often as required and enjoy the perfect results. You don't earn any money from the duds, only the successes.