I'm also going for tier 3. I don't think it'll be too hard, just a bit grindy since I imagine most of the money will come from selling stacks of stone from the mines and crafting any copper commerce guild requests for extra money.
As for mining for relics vs fishing... I actually enjoy the mining (though I think there needs to be more color contrast in the abandoned mines, though I wish I could hone in on more than one relic at a time. Maybe instead of "renting" the jetpack and goggles any time you go in the mines, they could be equipment that you actually have to make inventory space to bring with you, and you can work on upgrading them as well. Give the jetpack a glow in dark areas, increase the distance at which you can spot relics, increase how many relics you can hone in on at once, install an aura estimator that makes different relics glow a different color, etc.
On the other hand, I find the fishing system extremely boring and unsatisfactory. SDV's took some getting used to, but I enjoyed it with the right setup, but it definitely wouldn't be right for this game. I think the base system used is good, but it definitely needs some more work. It feels a bit more like modern Legend of Zelda fishing, but what Zelda has going for it is controller vibration, which is really great at giving subtle cues. I'm mainly thinking of OoT and Twilight Princess here, but regardless. Perhaps having more sound cues to mimic that would be good.