TK used to match from almost anyone, however in recent years it's a challenge instead, so you'd need some TK flights to make it work.
I'm going to say that since we're so late in the year right now, it's probably better to continue with UA for this year then switch in January. Some programs are based on calendar year, some are rolling, but because UA is through January, you both wouldn't qualify for Gold in UA and also wouldn't yet qualify for *G level in any other program yet by the time your current UA Gold expires.
As far as other programs go, yes A3 and TK have been popular for ease of *G but they wouldn't otherwise benefit you and aren't very convenient to use for award redemption either.
SQ, NH, and OZ seem to be your next 3 logical choices.
If you can get a credit card which credits to SQ KF as well, it may be worthwhile since I'm sure as you know SQ premium cabins generally can only be booked with KrisFlyer miles.
NH, I don't see it being terribly beneficial as NH Platinum (50K level) requires 25k to be on NH flights and doesn't give much benefit beyond *G from another carrier. Also keep in mind on international itineraries, NH only allows Round Trip or RTW awards, there are no one way awards so miles are a bit harder to use before expiration if not accumulating fast enough.
OZ can be another popular one as it's a 2 year membership year with a fairly low threshold, however if you're just barely getting the 50k a year now, it may take you over 1 year to qualify, so be aware that you'd be without benefits for a short time.
Regardless of which you chose, be sure to look at the types of fares you're usually flying on and their earning rates. For economy Mileage Plus is actually fairly generous, so you may find that what equals 50k a year in Mileage Plus may be 25k or even 12.5k in another program. If you're flying all business class fares on non-JV partners, then you'd probably be way ahead switching almost anywhere else since UA isn't your primary carrier.