Welcome back! Thanks again for playing and for the kind words! I try to keep updating regularly to polish the game and fix any bugs. At this point though, the game feels stable and bug-free, so I've been focusing more on QoL improvements and localization.
I agree that diamonds are one of the more difficult resources to balance progression-wise. I've done several playthroughs myself, and it often comes down to rng. In some runs I get lucky with slots/treasure chests or diamond spawns, and they end up feeling almost trivial. In other runs, I've had the exact same experience you described, where diamonds become a major bottleneck and I spend a long time actively hunting for them. I'm still looking for a good solution to reduce that rng dependence, possibly some kind of exchange system. I've considered lowering diamond costs before, but I don't think that's the right approach since players with good rng would end up finding diamonds even less meaningful.
As for Cobalt and Vermilion, part of the reason they become so common is Griddle. Their spawn rates in Farfields are mainly designed around Griddle's survivability. I also try not to add too many pre-Farfields ores to Farfields, which leaves me with only two metal types to work with. But I do agree they lose a lot of their value once you've progressed a bit. I'll be adding the ability to sell Cobalt and Vermilion to Bits & Bobs in a future update, and I'm also considering introducing a third metal ore to Farfields to help balance resource distribution.
Regarding older tools, I think it's quite common for early-game gear to be phased out as stronger options become available, but I like the idea of giving them more niche uses. Special achievements tied to older tools could be fun.
For power progression after unlocking Farfields, there were two approaches I considered. The first was to continue progression at a similar pace, where training Power 7 would take as long as (or even longer than) reaching the pre-Farfields cap. The second was to treat Farfields as a new chapter, where players basically go through an early-game progression curve again, with faster initial upgrades before things become more demanding later on. I went with the second approach because I felt players would be more excited to start exploring the early Farfields layers and mechanics sooner rather than later.
Thanks so much for taking the time to write such detailed feedback. I'm always grateful when players dig into the game this deeply ;), and comments like yours are incredibly valuable for helping me improve it.
P.S. Rejecting the Tanning Rack (Hardmode) is intended more as a challenge run than a fully balanced progression path. There isn't any progression balancing built around it, so it's extremely difficult. As far as I know, nobody has beaten Hardmode Farfields with Nix yet, and I'm honestly not sure it's even possible in the current version.