No matter what you set the tankless output temp to, the showerhead output is going to be ~105F, give or take a couple of degrees. At 115F it would be scalding hot. If you set it that high you'll have a lower gpm/higher delta-T through the tankless than at the shower head, and mixing in some from the cold side at the shower.
With 55F groundwater and a 105F shower output you have a 50F delta-T. At 5 gpm of shower head flow that's about 2500lbs/hr, so your peak draw is really (50F rise x 2500lbs/hr=) ~125,000BTU/hr.
The NRC98 takes in 180,000BTU/hr, so even if you assumed only 80% combustion efficiency at max-fire (it's actually higher than that) you'd have 144,000BTU/hr of output to support a 125,000BTU/hr load, which is PLENTY of margin. Then when you figure 2.5gpm showerheads really run only ~2 gpm unless you have unusually high water pressure, the only time you'll be bumping the thing up to anywhere near full fire is with 2 showers AND another significant simultaneous draw, or when filling a tub.
With a recirc you won't get very high net efficiency out of the recirc burns, since the incoming water temps are high and the volumes are low- it's short-cycle. If you go that route the push-button versions offer the least wear & tear, and higher efficiency, since you'll be feeding it cooler tepid-water from the recirc path, and you'll be using the hot water immediately rather than making it a separate burn.
The pressure drop issues are largely overstated as a problem. Where & when it's an issue there are relatively easy fixes that don't involve bigger burners.
The difference of min-fire output isn't enough to be of concern. It's primarily an issue of temperature regulation at very low flows, but anything under 20K won't have flame-out issues at low flow in summer the way some old-school 30K-min tankless units can. Most of them are pretty efficient at low flow min-fire, but the sweet-spot is usually at a somewhat higher fire than that where there's a bit more turbulence in the gases on the fire-side of the heat exchanger. Where you should worry about efficiency is at flows that are sustained and high volume, such as showers or tub fills, since that's what will show up in fuel use. Most low-flow draws are short-cycles anyway, often throwing away as much heat in the ignition cycle & flue purges as went into heating those 1-2 cups of hot water that you actually used. But most of the fuel is burned for the volume draws.