You just said get all to 15, then drop stuff on there that's from level 50? And what's useful to get to 15? And why? That's what I mean.
Getting to 15? You can honestly just spam stuff from the low levels (ingots/rings/cloth/etc). Every class has SOMETHING you can make that is basically ether just one ingredient or two cheap ones. For instance, I made lots of bronze/iron ingots and rings/plates for BSM and ARM, lots of salted fish for CUL, lots of cloth for WVR, etc. The investment for mats isn't that much (though you may want to buy shards from the MB) and you can speed this up quite a bit with Company-Issue Commercial Manuals 1-2 (+50% xp for 50-100k crafting xp) from your Grand Company Quartermaster and ether Helping Hand 1-3 from Free Company Actions (10-20% extra xp for crafting) or Squardron Engineering Manuals (20% extra xp for crafting for 120 minutes) from doing lvl 50 Squadron missions from your Grand Company. Just make sure you have plenty of crystal shards... you will go through quite a few. There really are no tricks required to get to lvl 15; you can literally do it the crafting guild hall if you want.
As for skills... That's just sort of how it is when it comes to craft skills for crafting. The stuff at 15 is the easiest and most important to get immediately, but the lvl 37 cross class skills are garbage except for Steady Hand 2 and Flawless Synth, so most of the "must haves" end up being at 50. It's badly designed and it's one of the things the devs want to address with the future crafting rework (that they haven't gone into yet, so might be awhile off or they might simply be waiting till next live letter).
To break down why ALL of this is important... for reference, Progress = Finishing the item, Quality = Chance for HQ, and Control = Stat for increasing Quality.
Rumination CRP 15: Gives you back CP for steps of Inner Quiet. This is great before you get Byregot's Blessing.
Byregot's Blessing CRP 50: This is key to doing high-end crafts. Build up stacks of Inner Quiet, then consume them with BB for an explosion of quality.
Ingenuity 15 BSM: Lowers the level of the recipe you're making for a few steps, making it easier to get Progress and Quality. Good for leveling.
Ingenuity 2 50 BSM: Like above, but better and used for tougher * crafts.
Rapid Synthesis 15 ARM: 250% progress but with a 50% chance and it's free. Combine with Steady Hand II and it can save your ass if you get hit with RNG too hard.
Piece by Piece 50 ARM: Completes 33% of missing progress. The more progress needed, the more you get.
Waste Not 15 LTW: Cut durability loss from 10 to 5 for 4 steps. Basically better than Master's Mend in every way.
Waste Not 2 50 LTW: The same, but for 8 steps.
Careful Synthesis 15 WVR: 90% of your normal progress gain for 100% accuracy. Useful.
Careful Synthesis 2 50 WVR: 120% your normal progress gain for 100% accuracy. A must have.
Tricks of the Trade 15 ALC: 20 free CP when your status is Good/Excellent. It's free CP when you don't want to make progress/quality, what more is there to say?
Comfort Zone 50 ALC: You spend some CP to get more CP back over time. 14 CP might not seem like a lot, but I assure you... it is when your doing high level * crafts.
Hasty Touch 15 CUL: Increases Quality without using CP, with low accuracy. Combine this with Steady Hand 1-2 and it's all you'll ever use to make progress until you get Hasty Touch 2.
Steady Hand 2 37 CUL: +30% success rate to all actions for 5 actions (so Rapid Syn/Hasty Touch go from 50% to 80%). This is KEY to doing anything consistently at the higher levels.
Reclaim 50 CUL: 90% to reclaim your mats if you fail a craft. Did you fuck up? Hit this and hope you get your stuff back.
Manipulation 15 GSM: Restores durability for a set number of actions. I consider this to be better than Master's Mend. Cheaper too.
Flawless Synthesis 37 GSM: Free progress. Completely worthless unless you use it with Maker's Mark, after which this no longer costs durability. Can be useful in some * crafts.
Innovation 50 GSM: I forgot to put this on my previous list. Gives you 50% more control over 3 turns. Combine this with Byregot's Blessing and you'll HQ stuff consistently.
Maker's Mark 54 GSM: This is basically the only post 50 cross-skill worth using. Combine with Flawless Synthesis for lots of free progress.
You can't use all of these all the time (you can only use 10 at most) but some you'll replace as you level. I generally use...
Byregot's Blessing
Ingenuity 2
Rapid Synth (but only until I get Rapid Synth 2 post 60)
Piece by Piece/Reclaim (both are good options, but I generally only want one or the other)
Waste Not 2 (But only until I get Manipulation 2 post 60)
Careful Syn 2 (though this is less useful once you get Careful syn 3 post 60)
Tricks of the Trade
Comfort Zone
Hasty Touch (Again, you get a better version of this post 60 but this one does take CP)
Steady Hand 2
To put it lightly, being a GOOD crafter involves a lot of investiment in leveling up other crafting classes. This makes it expensive to be a good crafter, but you make it all back when you start selling furniture on the Market Board.