I'm definitely in the minority here, but I disagree that Brussels is missable or not worth even a day. I've been to Brussels twice, the second time was last week. It's a city quite full of interesting things to see and do for people who take the trouble to do some advance research.
At present there is a wonderful temporary (until the end of August 2019) exhibition about the life of Audrey Hepburn, the actress and humanitarian. It's "Intimate Audrey" and is located in an easy-to-find building across the street from the "back" entrance to Galerie Royale Saint Hubert. Allow 90 minutes to see the exhibit in depth. The Galerie is a not-to-be-missed historical building full of wonderful shops and several cafes in the first ever enclosed "mall" in the world, built in 1847.
Also in Brussels is the European Parliament, a considerable number of wonderful Art Deco and Art Nouveau buildings and houses, the Atomium, the wonderful Grand Place, and many streets near to the Grand Place full of shops and cafes.
That being said, I thoroughly enjoyed the 2 full days I spent in Bruges and I could have spent at least one more day there. It's a completely walkable town with many churches, museums, lace shops, waffle stands, and more chocolate shops than you can count.
Your train arrives in Brussels at 1843, what I would do is spend that night in Brussels, check out of your hotel in the morning but leave your luggage there. Spend the day exploring Brussels, this is an instance where taking a HOHO bus would allow you a good overview of the city and you would have time to get off the bus several times to explore a neighborhood or area of your interest. Then collect your luggage and take a train to Bruges, it takes about an hour. Taxis are readily available outside the main entrance of the Bruges train station and are inexpensive.
Check train schedules with the Belgian train company. If your flight is at 1830 you could leave Bruges as late as noon and be at the airport three hours before departure. Trains do go directly to the main Brussels airport. It's possible you could get a direct train from Bruges or you might need to change trains in Brussels.