When choosing a plywood finish be aware that plywood can raise its grain severely with a water based finish and blotches easily with certain stains. Ideally, the bench with all its nooks and crannies is an good candidate for lacquer if you have a spray setup. A hand applied finish like Watco oil is another possibility. With any hand finish remember that getting into those little gaps will be challenging.
Wanting to add a little warmth, I chose a long time personal favorite: Osmo Polyx Oil. This German floor finish is made from various vegetable oils and waxes. You can hand apply the very thick Osmo in the manner of a rub-on oil/varnish finish. Wipe on. Wipe off. Build up three or four thin layers, and it’s a rich and very durable finish.
Don’t forget to add furniture glides to the four flattened legs. Being a heavy bench, use small HDPE glides so it can easily be pushed or pulled around.