for star best programs would be (in no order) ua tk a3

others will prob fill you in on other alliances and others will tell you to just skip the loyalty game and just fly with ultra lccs like iceland air and then use savings to pay for vacations and buy a priority pass membership (if lounge access is important to you). also if lounge access is important maybe a amex platinum is a good idea as the centurion lounges they run are the nuts (also do lots of research before getting any cc, if you just go willy nilly and sign up you could lose out on massive bonuses) also judging by your usage of the word 'commute', it seems like you'll have 2 domiciles so checked baggage will not be happening

you should research star gold itself and its benefits, and the benefits of ua tk a3 and figure out which one aligns itself better with your needs and wants. each has pros and cons, pros being:
-ua: low threshold to get you silver which gets you priority security, boarding, and free e+ at checkin. also all ua fares earn 100% elite status(but garbage award ratios)
-tk: star gold lasts you 2 years (and iirc, it's based on what you earn in any 12 calendar months, not one calendar year like ua) and low award redemption levels (but garbage award and elite earning ratios on some discount econo fares)
-a3: star gold threshold is really low, and even lower if you can make the 4(?) segments on a3 ever y year

also it is critical that you understand how fares on other airlines (and in the case of tk, even itself) compute into award and elite status. some ua fares earn very little on tk and vice versa. ua's is here, google "earn miles other airlines (tk/a3/lh)" for all the other star carriers
https://www.united.com/CMS/en-US/mar...ePartners.aspx

chances of upgrades on intl with the little amount of flying you are doing is very low, and even lower being that you're implying you're trying to keep costs down. therefore, disclaimig that i only know star programs, i'd say jus gun for ua and be happy that after just 25k you get 'priority' screening, boarding, and a e+ seat (2-4 inches more legroom and that's it). e+ is the closest you gon get for an upgrade. and 50k you get a free mediocre lounge in ewr but at least a lounge you can get free well shots from.

you also face a dielmma: put eggs in one basket and get something like plat with ua (where you have better chances for upgrades domestic) or go for gold with ua and try to get silver with another alliance (plat has just recently depreciated to the level of gold very recently with ua). i say eggs in one basket but that's me.

totally being that surly skater kid from an 80s movie: 8 roundtrips a year isn't a commute, a commute is more like 8 roundtrips every .5-2 months.

i'm sure others will give you points and tricks

fyi ua out of iah has $550 round trips to madrid, if you can find a way there (from lga? phl? ????). i have no clue about the ny market but i'd assume it's pretty competitive.
https://escapehouston.com/9972/cheap...rid-barcelona/