The Tx51 has more output than a standalone hot water tank- upsizing a mod-con for higher domestic hot water performance is almost universally a bad idea. That argument for upsizing to the TFT154 is utter junk. (Sizing the volume of the indirect for the domestic hot water load is what works best for 49 out of 50 homes.)
I'm not surprised that they would recommend primary/secondary even with a fire tube boiler- it's harder to screw up. With a higher-head water tube heat exchanger like the Tx it's still sometimes possible to pump direct, but I don't blame them too much for insisting on primary/secondary, since most boiler installers don't do math.
Of the TFT series the TFT154 has the lowest minimum fire (lower than their smaller boilers, which is curious), and probably the best choice of that series, assuming your heat load really is big enough. If your heat load really IS 30,000 BTU/hr @ 15F it won't be modulating once the outdoor temps are much above 40F, which isn't much warmer than the January mean temperature in Baltimore, and below the mean temperature of the 12 coldest weeks at that location.
Couldn't find data on the water volume of the TFT154. It's dual-port plumbing, burner output and turn-down ratio is remarkably similar to the HTP UFT 140W, which makes me wonder if it's another Kiturami export model under the hood. IIRC the UFT series are all on the order of 3-3.5 gallons, which isn't exactly a huge thermal mass.
"...constructed to short cycle!" reads like pure marketing BS. Just because it tolerate some amount of short cycling without losing much efficiency doesn't mean that there are no down-sides to running it that way.
Back on 3 November you wrote "I'll note that using 3800 HDD and a delta 70F on design day does put me at 49K BTU".
There is no location in MD where you would have a 99th percentile design condition with a 70F delta between the outdoor and indoor temperatures! Even up in Cumberland the 99% outside design temperature is +10F, so at a code-min 68F indoors in Cumberland you'd be looking at a delta of 58F. If 49K was the load at a 70F delta, at a 58F delta you'd be at 58F/70F x 49K= 40.6K. In Baltimore the design temp is +15F at the airport, +17F in down, so in Baltimore you'd be looking at a design load of 37K or less.
Run your fuel use load numbers more carefully, and run them at your actual 99% outside design temp, not the coldest low temp, since the 99% temperature bin is a more reliable indicator of the average load. If the mod-con model won't modulate well at your average winter load, you will be better off with something that modulates a bit lower. Calculate the design load carefully and aggressively- don't try to put a thumb on the scale for oversizing at that point, since that will lead to even greater oversizing when you're further along in the process. Also know that there is an inherent bias to oversizing just from using the nameplate efficiency on an old oversized boiler. Don't try to correct for that- run the numbers as if it were right sized and brand new, then you'll have confidence that the result is a firm upper-bound on the actual 99% heat load, and you can then adjust accordingly. Then re-run the load numbers at 35F or 40F outdoor temperature rather than the 99% outside design temp and see what you come up with. If it's 15,400 BTU/hr or less @ +40F you want a smaller boiler than the TFT154.