Sometimes you have to spend money to make money.
Reading all your questions and complains about crafting over the past months almost makes me want to tell you to drop crafting alltogether because you obviously dont understand this very basic principle that is infact important when it comes to crafting.
Its not that I dont understand your position, I'm trying to do as much as possible myself instead of just buying everything from the marketboard, but I will do so on occassion if I consider spending gil being the actual cheaper option in the long run.
Looking at your example here:
The demimateria you need requires that you desynth glamour or crafter gear - DoW/DoM wont yield fieldcraft materia. In order to get those you'll either have to
(1) buy them from a vendor, potentially for more than 8k
or
(2) craft them, using materials and your time, that if added up at marketvalue and factoring in the RNG might end up costing you more than 8k aswell.
Not only is it probably more expensive to get this demimateria from deynth, even if they change it up with 5.1 so that you wont need to level it anymore - RNG is still gonna be involved, so you might end up having to purchase (direct waste of gil) or craft (indirect waste of gil by wasting materials and time) multiple items instead of just getting the demimateria you need.
In addition to that its also hindering your progress right now, correct? Potentially losing you money indirectly again because you're being stuck at this part.
Being cheap is okay - but you should try to be smart about it, too. Blindly refusing to buy things might cost you more in the end...
To give you an example here:
I'm making my money by crafting endgame-items - gear, food, potions, that stuff. My time equals money here, so I'll look at the marketboard to figure out wether its worth my time - that I could spend crafting - to go out and gather the materials myself. If I have an hour at my hand to spend with "making money" I'll consider how to spend that time best. If the materials are fairly cheap on the market, I'll make more money in that hour if I invest some of my gil instead of my time and just craft the full hour. If materials are extremly expensive, I'll go and grab them myself.
Serious advcie here: Stop being cheap by default. That will only cost you.