As far as the T-shirt question. The interfacing really helps with the cutting. I would fuse interfacing to the inside of the shirts behind the decals/ designs you want to use, cut out your blocks then stack those nice & neat to save for later- then that step is done when ready to sew. Pizza boxes work nicely for keeping blocks straight. Another use if there are a lot of T-shirt type fabric is to make (T-shirt) yarn from them ( lots of video tutorials to make/cut the yarn) you can use that for rugs, woven, hooked, crocheted/ knitted.....any number of things, even embellishments.
when I was ( overwhelmed) by the bags of clothes from my mom passing I did deconstruct many articles of clothing.
The easiest- fastest projects were memory pillows. A few members of the family had favorite sweaters, shirts, sweatshirts that they had either bought her or just remembered her wearing often. I sewed the necks closed, folded sleeves & stitched them down ( a couple I actually stuffed- made into a hug) either stuffed them or inserted a pillow or pillow form. I put a label on them personalized for each person ( daughter, granddaughter, nephew etc) saying “ this is a shirt I once wore, when you hold it know I am here....”
sewed closed the bottom. I could make a few pillows in a short amount of time.