You could try lots of things. Changing the rhythm pattern is one, but you could also try things that work with tone (timbre) and dynamics too. Here are some ideas that come from guitar, but I think should work on ukulele too:
- Palm mute. Creates a softened, quieter sound. You can increase/decrease the amount suddenly or gradually.
- Change the position where you strum across the strings. Closer to the bridge becomes brighter, away from the bridge becomes softer.
- Change speed and direction of the strum. You get a kind of brushing effect with a slow strum. Usually you strum down on the down beat, but strumming up creates a subtle difference in sound.
- You can slide into the proper chord from a few different position - above or below by one or two frets. Just move all your fingers in one locked position at the same time. You can place the slide at different points metrically - full beat before the first beat, half beat before beat three, etc. Try to see if any work with the song.
- Mix in some arpeggiation with strumming the full chord, or maybe a tremolo effect by alternating two strings rapidly.
- Play the chords in various positions up the fretboard. With 8 bars per chord you have a lot of time to move through two or three positions for each chord.
With all of these you can control how much change you add to a basic pattern. This could mirror the tempo increase the song already has. Start out with a little bit of variation to a basic strumming pattern, then add bit more as the song progresses.
Another approach is to add space. Can you add some rests or hold a chord for a longer time? Maybe only add the changes or extra ukulele parts when the voice comes to a rest. The possibilities depend on what the vocal part is doing.