Desynth is hot garbage, glad I didn't spend money on leveling my desynth alchemy. This expansion. Pretty sure I spent like 15millions in it over the years, and it never really made any money since ARR but I always kept it up "in case of", but this expansion I was like nah fuck that I'll just desynth books I loot and not vendor shit. Glad to see it go, although it'll probably mean you won't be able to desynth dungeon gear anymore I guess, so the utility of desynth is going to be super low. Which is fine, it wasn't that useful to begin with, I guess I got a bunch of GSM and WVR crystals over the years from dungeon gear but I could just buy them anyway.
On the crafting/gathering leveling question, there's a few methods to go with there.
First thing is you'll want 100 leves to get started seriously, and honestly leves are gonna be quite a limiter at first. Then you take all crafters to 15 doing 1 of every craft strategy(1st time xp bonus for crafting new items) using the guild vendor materials. This gives you plenty of cross class skills. Next step is getting the good higher level ones, that should be something to aim at but not necessarily a high priority. WVR 50 is mostly the only really big one left, although ALC to 50 is great too. Everything else is kinda whatever, until 54CUL which is also great, and then you want 50 LTW before you hit 71+ on other stuff. I would recommend getting everything to 50 first though so you can ditch all the ARR gear and materials, and then everything to 60, to 70, to 80, basically in order so you can keep the gear the same for everyone and stuff. I'd also recommend doing CUL on its own in one go, since it's a nightmare on your inventory.
On gathering, wether you want to them along or not is a matter of taste and funds. If you lvl gathering along, you can gather materials you need to lvl crafting so that saves money, but lvling gathering this way is also really slow. The faster way is doing leves, but that directly competes with your crafting leves allocation. Up to you what you do there, however I would heavily recommend against lvling FSH at the same time, FSH is a big fucking mess of inventory space and should be kinda done on its own. It has very few useful mats for crafting anyway, and you can generally buy them if needed. Once you're past 50, you don't need leves anymore for them though and it goes pretty fast with collectables. However I'd recommend lvling crafting first so you can make yourself gathering gear, as otherwise there's some annoying "holes" in the gearing process between expansions(big jumps due to the gear progression at lvl caps).
Don't forget to do your daily GC turn ins(one HQ item every day for big xp and some GC seals, which you can then use to buy xp manuals), beast tribes are also great, Ixal is a bit weak but still pretty nice to do for crafting, Moogle are very good for crafting and Namazu are good for both crafting and gathering. Once you get to these tribes you can kinda do slow leveling over time for free if you want.
Also, especially for ARR leveling, if you don't have much funds to get started, it can be good mass crafting stuff that sells decently like ingots and cloth and what not, and sell that. Use food and GC manuals and just spam the crafts. If they sell in large quantities, quicksynth can be good, otherwise do manual crafts and set up a AHK or keyboard macro to repeat the process over and over while you afk. Just go craft in your house/appartment or in the barracks in your GC or your inn room(I think you can craft there now). Remember however that gear breaks and once it breaks you'll fail all your crafts, so you can only do so many crafts afk. Quicksynthing especially is terrible for gear durability. If you go this route, make sure to buy/craft new sets of gear everytime you spiritbind your gear, convert that into materias and sell these or start saving them. It'll be additional money.
Overall, leveling crafting is a lenghty process unless you're super dedicated, or have a lot of money. You should take it easy, set realistic short term goals and work on it every now and then. The payoff is... well it is what you make of it, but nothing really beats crafting for making money endgame. You can literally make hundreds of thousands every 30-40secs craft just by buying mats, converting them and selling the finished products.