It has never taken me longer than an hour by taxi from Suvarnabhumi Airport to the Conrad, usually 45 mins
The regular taxis on the lower ground floor as you exit the arrivals hall are fine. They are waiting there and you don't have to wait and find them as you would a Grab. The price is similar, Grab is sometimes even more. Make sure you have cash of course, both for the meter fare and the tolls.
The public transport options work but are really not worth it for the small saving. It's complicated and cumbersome especially with luggage
Arrive in comfort (and avoiding the stress, heat, sweat and effort) and just take a taxi