If you take the flight to Sde Dov and it lands at 20:45, you will probably get out of the airport by 21:30 and a taxi to Ben-Gurion will get you there at 22:00. This is 3:20 in advance of your departure so it is a feasible option.

However, I recommend taking the 19:30 flight if you are worried about delays, etc. If it arrives at 20:20 at Terminal 1 you will probably get to Terminal 3 around 21:00 so you will have over 4 hours until you flight departs - but you will be more relaxed !

I suggest that you go into the airline's web site and check if that flight actually exists.