Water chemistry help

Hello all,
I'm new here and have benefitted so much from reading everyone's wonderful advice. I think I have now read enough to formulate an intelligent question...thanks very much in advance for any help you are able to offer!

I have kept fish for years with good success which I now know was kind of dumb luck because I never paid attention to water chemistry other than pH and ammonia/nitrogen cycle stuff until my most recent setup! (yikes)

My goal is to have a low-tech planted tank with mostly epiphytes and lower-light plants like Amazon Swords with angelfish and cardinal and rummy nose tetras. I have kept all of these species before but what's new for me are the plants. I bought an Eclipse 37g aquarium kind of impulsively off of Craigslist because I had this system in the 29g form years ago and found it a pretty easy and neat-looking. The tank had been in use and the filter media/biowheel and the gravel were "live" and I initially set up the tank with that gravel and two young guppies that the seller gave me when I picked up the tank. I added some driftwood, quartz rocks from around my house, some sand on top of the gravel, and several plants and all seem to be growing well (new leaves on the swords, little melt). Tank was running with this setup (also added some API QuickStart and NilocG Thrive root tabs to the gravel) for about 4 weeks with 1-2x week 50% water changes to prevent algae. I thought I was in great shape because I use well water (goes through a water softener) and the pH out of the tap was 6.8 - perfect for the species I wanted to keep.

I went to my LFS around this point to get start stocking the cleanup crew (bristlnose plecos, otoclinclus, and amano shrimp) and they tested my water. Ammonia/nitrites/nitrates all fine but my pH was 8.4. They sold me some discus buffer (didn't work, pH remained high). I then broke down the whole tank, removed all the sand, did the vinegar test on all the rocks (none reacted), and set everything up again. Measured pH the next day and again got a very high reading (8.2+). This is when I started learning about KH and GH and how the pH of water changes when exposed to air.

Current tank parameters (tank's been running for about 8 weeks now):
pH: 8.4
GH: 1
KH: 12

Tap water (from well after Na+ water softener, after sitting out for 24 hours):
pH: 8.4
GH: 1
KH: 7

Well water (before softening, sitting out for 24 hours - have never put in tank, just for reference):
pH: 6.0 (could be lower)
GH: 7
KH: 1

Currently, everything is alive but I think I need to make some changes before I stock the angelfish and the tetras. I know my GH is too low and I think my KH is too high. Here are my questions:
1) Any thoughts on what could be raising the KH of the tank compared to the tap water?
2) I can easily install an RO system near the tank and try again with RO water that I remineralize - is this what people would recommend in my situation?
3) While I'm investing in an RO system, I have a sneaking suspicion that in the long term, I'll be unhappy with my current tank and will want to switch to a 40g/55g long to more easily maintain my plants. Will I be happier in the long run with a planted tank if I go with a longer/less deep setup?

Sorry for the very long post and thank you so much for any insights you are able to offer!
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