I mostly go downstairs to the taxi rank, as I don't know how to travel light and it's a hassle to lug luggage around You get a number which gives you an allocation for the taxi. The cost should be about 600 baht.
I have used the train twice, without luggage, to go to meet people at the airport. It's very straightforward. From the airport you travel to Makkasan, then follow the crowds to walk a fairly short distance to Phayathai station. Get the train which goes to National Stadium and change at Siam station and get the train from there to Saladaeng on the Silom line, which goes to Bang Wa. Someone please simplify it if I have made it a bit complicated.
If you are reasonably fit and can handle the heat, it's a walkable distance to Ibis Styles from Saladaeng station.
The good thing going from the airport is that you should be able to get a seat. Going to the airport can be a lot more difficult because people are usually packed in like sardines.
The SIM cards are useful if you want to order a Bolt or Grab taxi or use Google maps, etc, otherwise just use the hotel WiFi.
Vinapu, can you please tell me how much you pay for your SIM card? I have a separate Thai phone, keep the SIM card in there and top it up when I arrive. It would probably be easier just to use one phone which can take both cards.