You can review the weekday and weekend SARL schedules here:
http://airportraillink.railway.co.th/en/index.html
I think the Express to Makkasan may still be out of service?
Traffic, if considering a taxi, should not impact your journey given the days of week, time of day.
While there is an overhead enclosed walkway in/around Asoke interconnecting the Skytrain (BTS) and Subway (MRT) you may still have an unsheltered walk near/down Soi 18. It is rainy season now so remember to pack waders and an umbrella if you plan to walk from the MRT/BTS.