Yes, I suggest you somehow support your sides of the raised garden. I have driven rebar posts in the ground right in front of each side board and that helped. Not only might the sides bow out due to weight of soil, they might bow out over time as they warp. Good luck

Are you going to add any 'pedestrian crossings'? 32 feet is a LONG way around to get to the other side for row planting.

Depending on your agility/mobility you might want to add a step plus some sort of 'grab pole' to steady yourself while stepping onto the bed itself, and have several down either side.

And having walk-ways will both divide the bed plus stop the soil from compacting. That way you can leave a long grow crop, such as onions or potatoes, in peace in a block, and still work on the catch crops such as radish, lettuce and baby carrots.

If the bed is for flowers, same applies - particularly if you need to set/lift bulbs or perennials.

If you're planting shrubs - think pruning, weeding, mulching, and feeding - how? You can't do all that work from the surrounding pathways because the reach is a little far (unless you are blessed with more height than someone as vertically-challenged as I am! I can manage a two foot stretch but I wouldn't like to have to work 32 feet while stretching so far.)

Even though it's only 18" deep you're going to get quite a bit of outward pressure with a 5 foot span. My guess would be you'd want to connect the sides at 4 foot intervals at the minimum. This could be useful in grouping your plants as well.