Are you just venting, or would you like some help?
That sucks that you are having issues and regrets.
I can start by telling you that if you have a tankless water heater, the biggest issue folks have with those is setting them too high. Cranking them up to 125f degrees like a regular tank WH and mixing hot and cold water at the fixtures makes them freak out. Set it to what you can handle using only the hot side of your fixtures. I like a 104f-106f shower. Some can handle up to about 110f. You'll have to elaborate on any additional WH issues you're having.
What do you mean about the onboard tank and water pump? Don't use in general, or don't use for the water heater? Either way that's nonsense.
What about your sink won't work? The faucet, the drain?
Sorry about your toilet pedal.
Good luck!