Glorified-tankless combi-heaters pretty much fill that bill.
The RayPak XPak 85 is a finned water-tube boiler, 68,000BTU/hr out at max fire with a 4:1 turndown rato. With 17K at min-fire you'd be counting on it's hysteresis or bumping the temp up a bit to keep it from cycling on zone calls. It can run in condensing mode, but doesn't have to.
The Rinnai E75C combi unit is pretty similar, 69K out max, 17K-in min.
The Navien CH-180ASME also modulates down to 17K-in, with a much bigger 150K max, but the reputation & technical support for Navien in N.America is less than stellar. Web-forum complaints abound, but more often the not the issues are related to incompetent installation. Some folks do just fine with them for years, others... It's attractively cheap though ~$2K USD at internet pricing.
These finned heat exchanger water tube boilers all have condensing potential, but only test out in the low-90s range on AFUE type testing, and are $500-1000 cheaper than most small mod-cons. (The Rinnai might be a bit higher than the others.)
At 2gpm it only takes about 70KBTU to heat water from 35F to 105F (showering temp), so you'd be able to take endless showers even running it in combi-mode, but with the indirect operated as a priority zone you could even do higher GPM for reasonable amounts of time. I don't quite get how you think it'll need 100-120K to support your DHW loads WITH the indirect. Most homes do just fine with 40 gallon indirects and a 50K boiler, unless they have an oversized tub to fill.
A 6" flue is kinda overkill for any of these, and even though they're forced draft you'd may run into issues if you have lateral runs more than a few feet with such an ovesized flue. Even active flue purges aren't guaranteed to be effective if the flue's total volume is way oversized for the BTU rate of the equipment.
Which non-condensing modulating boiler did you come up with?