Resource prices should go up a bit soon, they tend to get a little more popular on weekends while higher levels are busy raiding.
Still I recall good old times, when it was so easy to throw millions into investments without much second thought.
This could just be a side effect from leveling up. Early on the gold from quest mats is amazing (and it still is), but as you level up you start needing a more gold so it feels like less in comparison. At some point you'll probably even transition into "wow, these mats are so cheap, I can buy hundreds!", even though the prices haven't changed.
2. What are the best ways to earn gold in the midgame? (20-30 levels)
I never liked doing precrafts for gold at low levels, so if you want something you can sell to heroes I'd recommend working towards...
1. Music line! There's a couple popular precrafts in music if you happen to proc some (great harp, great nordic lute, great hellhound, etc), and level 39 twilight flute is good-ish for selling to heroes (3m base price, maybe 40 min craft time, good precraft). Getting to twilight flute is relatively easy, the only tricky item along the way is draconic sound (precraft is great silvered flute).
2. Heavy boots. Level 45 azure boots (8.3m base price, 90ish minute craft time, no precraft) is another popular one to sell to heroes, but I don't think anything else on that line is particularly popular and you'll need to get past 2 flawless precrafts to unlock the boots.
3. Precrafts are a good option later on when you've got higher level workers and more mastery but might be tricky at lower levels. Personally I only liked doing them when I wanted gems but some players like it for gold too. But if you're relying on fusion it'll be a big bottleneck to how much you earn in a day.
4. Always be fusing! Even if precrafts aren't your main source of income, if you don't need your cauldron now you can throw in something to sell. For example, if you put 1 good bone reaver with 1 good nordic lute, the result will be a great of one or the other (the goods cost maybe 100-200k each, a great of either fetches about 1m gold or 1-2 gems). There's tons of other items this works with too, the precraft list aks47u mentioned is a good place to look for other options.
5. When you feel ready, try to work away from the common line blueprints. Boots/flutes/precrafts are nice as a stepping stone but you can find some much more profitable BPs out there. Dragonfire potion from raid comes back in late October, Ocarina/Balmung from packs, KoS from dwarvish chests, goddess tear from magic, couple icky primals, and a few others are big game changers (but much harder to unlock than the common BPs).
Anyway, my usual advice is to try it and see. There's lots of viable options out there, and different players prefer different things. If you start with one strategy and decide you don't like it it's easy to change to something else later