Very hard water (268ppm) - dosing help

Long-time lurker here! I’ve been slowly revamping my laundry routine and already seeing major improvements (no more musty towels or smelly washer!) by increasing dosing, running longer wash cycles, and adding citric acid. We have very hard water (268ppm) and I could use input on how to select and dose a water softener (a whole house system is on the wishlist but not feasible atm).

Current routine: Tide w/ oxi or clean & gentle powders (filled to 3rd or 4th line depending on load size) and 1 tbsp of citric acid in the rinse drawer of an LG front loader. Sometimes I add DL to the drum. Will be using free & gentle liquid + FEBU for darks when it’s back in stock. I run the towels cycle on medium spin, warm, extra soiled (1 above normal), and 1 extra rinse.

Outstanding questions: Should I be using STPP, Calgon, or Borax and how should I think about dosing? I am leaning towards the STPP as I think I read that it’s better for baby laundry (hopefully I’m not imagining that). I do have a full box of Borax though and wouldn’t mind having a way to use that up. Lastly, how should I revise my detergent dosing if I were to implement a water softener? Thank you!!