How can I easily compare the affordability of two departure airports with respect to a specific set of airports I frequently fly to?

How can I easily compare the affordability of two departure airports ...

By looking at prices on Google Flights for durations, dates & booking classes that are representative of your specific flight plans, populating a spreadsheet, and looking at the results.

Sorry, I'm not trying to be snarky here. Google Flight's "price on calendar feature" and specifically the prize graphs are great tools to effectively display a large set of flight prices and quickly distill it to a single number. I just looked up SFO/SEA to TPE/HKG/MNL for Jan + Feb, and it took me less than two minutes, a lot less than writing this answer :-)

If you want to get really fancy and find the absolute lowest price for both airports at the same time, you can use ITA Matrix instead, but it'll take a you a few minutes longer.

FWIW: I didn't see any meaningful difference in my quick research. Differences seemed less than 10% and just "noise".

There is more than just price to pick a "preferred" airport.

1. SFO serves way more international destinations than SEA, including all of your targets (SEA currently doesn't serve HKG or MNL) so it's better for non-stop.
2. SFO is a hub for United, which serves all of your destinations. So if you fly a lot (or some premium cabin) you would be much more likely to accumulate benefits and status. While the airlines have devalued their loyalty programs a lot in recent years, it can still be fairly valuable in the right circumstances. SEA's only hub airlines is Alaska, which is North America only (plus Costa Rica).
3. Getting to/from the airport: That will depend a lot on where you live relative to the airport and/or public transit, but it's often a non-trivial part of the overall trip cost.

Needless to say: housing is substantially more expensive in the Bay Area, so unless you fly a whole lot AND pay everything out of your pocket (and not on company/business dime), the difference in actual ticket prices will not make a meaningful difference and it feels like the tail wagging the dog.