Size the indirect for the hot water heating load (sizing it for largest tub-fill is about right), and run it as a "priority zone". With 74K of boiler output you can run a 2gpm shower FOREVER, (that's more than 2x the burner output of a typical 40 gallon gas-fired standalone tank!) but if you have a standard-sized tub that you want to fill at 8-10gpm you'll want at least 30 gallons of tank,i f kept at 140F. You can go somewhat smaller if using high storage temps. (Using a 26 gallon ErgoMax and "sharing" output with radiation you'd want to run it higher than 140F during the heating season.)
The length of the baseboard determines the water temp required to deliver the load at the design temperature, which is going to be well-under 180F, even with the updated 134' fin-tube length. The fact that you've been running a tankless coil, at your annual fuel use it means that your space heating load at the 99% design temp of about +5F is well under 70K, and could easily be under 60K. So it's likely that the highest temp you'd ever need on the baseboard is about 155F, and if you insulate & air seal you can probably bring it down to 145F at the outside design condition.
With outdoor reset control on the boiler the average will be much lower. By running the baseboard at lower temp means the distribution & standby losses to the semi-conditioned basement will be lower, and the system-efficiency much higher.
On an old-school boiler you'd have to add in some near-boiler plumbing to protect the boiler (and the flue) from acidic condensation, but in the MPO-IQ84 that protection is internal to the boiler, no extra system design necessary on that front.
DO run the FSA calculator using your fuel use and the fact that it's an embedded coil (==massive shoulder & summer season standby losses) and let the tool auto-adjust your heat load estimate. (When I run it using your boiler size using Hartford with an older design temp of -1F, even plugging in 1300 gallons/year it's coming up with heat loads under 50K, which is still credible.).