The research is a bit mixed on when and why to take a DOAC (direct-acting oral anti-coagulant). Some think all AF patients have a raised risk of stroke immediately, some recent research says only after 24-48 hours in AF. The CHA2DS2-VASc is pretty solid as a predictor, although someone with AF should probably be at a 1.5 just for that alone...my personal opinion. If you are a bit nervous, ask your cardiologist if you should take maybe one or two children's ASA, 81 mg each day, one morning, one at night. See what he/she says.
Many of us take 200-600 mg of Mg daily as a prophylactic against AF. For some it seems to work, while others say it doesn't seem to have helped. The literature and anecdotes are replete with triggers of all kinds.