I am fairly comfortable strumming along a song with 80 or more BPM, however for songs lower that I struggle to keep up. How should I practice?
Strum twice as fast, but miss actually playing the strings on the 'off' beats. That keeps the arm moving at a pace that works for you, and actually, it's what a lot of strum patterns, fast or slow, will incorporate anyhow. So there may be a real sounding strum every down/up/down/up, but because there's a better momentum, it's easier to keep time. Think 'ghost' strums. And there's no reason not to include some of those ghost strums in actual sound.
At slower tempos, the time between beats can be hard to feel. To help develop the feeling, practice with a metronome. But rather than setting the metronome at 80BPM (or slower), set it at 160BPM. This will fill in some of the space between beats, so even though you'll still strum at 80BPM, there will be an "extra" click each beat to help you stay on track.
My suggestion is to do the opposite of what players do when trying to learn something technically challenging, which is start slow then gradually speed up.
You’re comfortable at 80bpm. Pick a handful of strumming patterns you’re comfortable with. Have them be rhythmically varied with different combinations of quarters, eighths, sixteenths and triplets too. Start with one, play it a few times at 80 then slow the click by small increments of your choosing. The idea is to train yourself to accurately fill in the blanks between the clicks with your chosen rhythm. Before you start playing count the subdivisions out loud to the click: 1+2+3+4+, 1e+a2e+a, 1-trip-let 2-trip-let, etc.
Work your way down to the 70’s then the 60’s. Remember to relax, it’s very important at slow tempos!
If the slow tempo is very regular, you can subdivide it in your head to predict where the beats will land. If it is, say 4/4 time with 𝅘𝅥 = 40, imagine 8th notes which are 80 per minute, or 16th notes that are 160. Say you wanted to strum twice per measure: every other quarter note. In your head you could count "strum!-2-3-4, 1-2-3-4, strum!-2-3-4, 1-2-3-4". This will eventually become internalized so you don't have to think about the counting.
Use your body. Sometimes rhythm players "air strum" their hand in a faster tempo, but only every other or every fourth strum actually hits the instrument. Or they make tiny "pseudo-strum" gestures during the silences, in time with imaginary beat subdivisions. The "-2-3-4", "1-2-3-4" counts could be accompanied by movements of the strumming hand, like it is about to strum but doesn't actually strum.
Instead of, or in addition to relying on this internal rhythm or extra movements, you could also pay attention to any other instruments which are subdividing the meter with faster notes.