To take a stab at getting the hydronic zoning "right", at a minimum you'd have to calculate the total volume in the PEX on each zone, and make sure that it's sufficient to keep the boiler from short cycling at min-fire. Half inch PEX has only 0.92 gallons ( 7.7lbs) of water for every 100' of tubing. At 8" o.c. tubing spacing a 400' master bedroom would have only ~600' of PEX, or 46lbs of water.
Assuming the combi boiler runs a 7F high/low differential around the output setpoint temp, that 46lbs of water takes only 7F x 46F= 322 BTU to heat up, and at 19,000BTU/hr of output that only takes 322/19,000= 0.017 hours, or 0.017 x 60= 1 minute to heat up, which is a fairly extreme short-cycle. Ideally the minimum burn time at low low output temperatures using the outdoor reset function would be 5 minutes, but 3 minutes wouldn't be a disaster. With 1300' of total space on the first floor I'd hazard that you have at most ~1500-1800' of tubing, or 115lbs or more of water, which would deliver tolerable minimum burn time as a single zone, unbuffered.
Run the math on the tubing of the three-zones as-configured, then how to combine them to deliver at least three minutes of burn on their thermal mass alone, which would allow you to run 100F or lower water temps for high-90s efficiency when it's not all that cold out. At 8" spacing it's 1.5x ratio of tubing-length to floor area, and 7.7lbs/100' of length.
In the basement you'd be far better off with panel radiators than with baseboard, to keep the boiler from short cycling. It only takes about 20' baseboard to deliver the ~6800BTU/hr at the 99% design temp with 140F water, but it's output at 120F is only ~4200BTU/hr, and you'e have 15,000BTU/hr of excess heat being dumped in. Assuming 50' of 3/4" copper on that loop (including the fin tube itself) you're looking at 10lbs of water for thermal mass- the thing will short-cycle like crazy with burn times well under a minute when only that zone is calling for heat.
Using a condensing tank heater the minimum burn times are determined by the thermal mass of the water in the tank, which is substantial, and it's internal controls. With a heat exchanger for the heating loops you can micro-zone it to your heart's delight, and it doesn't affect the math.
But at Seattle's utility rates, at a lower installation & operating cost, heating the main zones with ductless mini-split heat pump technology may make more sense, probably a 3-4 head 1.5-2 ton multisplit. Take a look at the room-by-room load numbers. Something like the Mitsubishi MXZ-3B24NA or Daikin 3MXS24JVJU or Fujitsu AOU24RML1 can handle three zones using a mix of interior head/cassette types, and can deliver over 27,000BTU/hr of heating. With mini-duct cassettes it's possible to "share" the load between two adjacent low-load zones. I'm sure the main zone has a heat load over 7000BTU/hr, and maybe the master bedroom too(?), which is enough load to rationalize it's own separate zone head. to get four-zone systems you have to bump up to a 2.5 or 3 ton compressor, which is oversized for your heating loads- enough to affect efficiency on the high end. Ideally the heads would be sized no more than 1.5x oversized for the individual zone, an the compressor no more than 1.5x the total heat load, which is why a 24000BTU compressor (typically 30,000BTU out in heating mode) is a better choice if you can make it work. Installation costs will vary, but it'll be WAY under the cost of a professionally installed radiant floor solution.
In a Seattle climate better class ductless technology delivers over 10,000 BTU/kwh, and you have some of the lowest electricity prices in the US. Like politics, all utility rates are very local, but last I looked natural gas in Seattle was on the order of $1.20 therm (100,000 BTU source-fuel, 95,000BTU if burned in a condensing boiler), or about $12.60/MMBTU (plus the electricity for pumping costs), and electricity was about 9-10 cents, which used in a better-class ductless costs $9-10/MMBTU.
What you'd be giving up is the barefoot comfort on the coldest days, but unlike ducted heating solutions, ductless systems are variable speed and very quiet, delivering VERY stable room temps, and are in no way comparable to hot-air furnaces. They provide super quiet high-efficiency air conditioning too.
For probably more money than ductless or radiant you could go for a 2-ton Carrier Greenspeed variable-speed ducted heat pump system for comparable comfort and efficiency, but you'd probably need to go with all new ducts, but I'm not sure if you can micro-zone it the way you can with a ductless approach.