The last SARL trains from SBIA to Phaya Thai depart at 23:47 and 00:02.

The last BTS train (to Bearing) stops at Phaya Thai at 00:21.

You could not connect from the last SARL to the last BTS.

The BTS has extended their early morning/late evening operating hours recently to 05:15 ~ 00:52 (Sukhumvit Line, first train out of Mo Chit, last train into Bearing).

The BTS schedule is probably the more reliable of the two; there have been complaints on Thai social forums re: SARL "cancellations" both early and late.