Thanks for the education. He did look at running it out the wall beneath the deck, but my windows and doors seem to be preventing this. I have a wall of windows and sliding glass doors on the back, then windows and another deck on the west side of the house. While there are no windows on the east side, the crawl space on that side is difficult to get to. It’s really crazy the way the house is built.
Most direct vented (both intake and exhaust piped to the outside) condensing appliances will allow as little as 12 inches from any door/window because there's no pressure displacement (Air mixed with that exhaust won't be sucked into an open window to displace outgoing exhaust)
The pitfall in your case is steam from the exhaust being visible outside a window or glass door on colder days.
Power-vented (vs direct vented) appliances only vent exhaust, the intake is inside the home which will suck air back in through a window, those vents usually require 3 or 4 feet from windows/doors.
As for electric tankless, don't do it, trust me, you'll probably need a sub-panel off your home's panel, and the lights will dim any time you use it, might as well go with an electric tank type, which will require a 40 amp circuit vs 100+ amps for a tankless. either option will require an electrician, but the 40 amp/tank option is less money.
I know it may seem trivial, but the way your old tankless was vented was a serious hazard, especially where it sounds like it was power-vented (intake in the same room as the tankless and furnace), I'd also make sure there's a CO detector in the mechanical room with the heater. If the tankless was running while the furnace was heating, CO was being sucked back in through the furnace's vent hood to displace the tankless blower's negative pressure.
The timing of your install was an "ouch" (Saturday, supply's closed), but I'll wager your plumber will call tomorrow with a pretty close explanation to what I just said.
As for a "standard efficiency" draft vent tankless, not sure they exist, but if they do they likely require a wait to be shipped, a very uncommon item. the alternative would be an ordinary tank type, but the entire PVC vent going all the way to the connection to your furnaces common vent would have to be replaced with galvanized pipe.
Your two best options, IMO, are electric tank, or direct vented tankless.