Have visited Sri Lanka on awards many times--was born there. A few points:

January will be noticeably cooler (less humid) than April, especially around Colombo. Also the whole place shuts down for the local New Year which usually occurs around April.

For *A, SQ via SIN and TG via BKK are options.

For OneWorld, it's CX, MH, RJ, with UL joining soon as noted, plus EY through AA.

For Skyteam, KE now flies nonstop from ICN.

There's a lot to do based on your specific interests. Be prepared to watch your life pass before your eyes if you plan to do much intercity driving!