(FYI I edited your post to remove the link and put in the picture instead)

So, the consensus is that harnesses can be pretty dangerous for rabbits. If they spook or struggle while wearing one, they can injure their back. If a bun is really well trained they can be OK, but there are safer alternatives.

The backpack looks fine, the main concern would be chewing through the mesh since it’s fabric. Also if it’s hot, the bun would be in the sun and could get overheated. I know some people have pushed their buns around in pet strollers that are made of similar materials, and some buns do seem to like it. A lot of buns are “home bodies” though, and tend to be pretty afraid outside or in a new area.

A good option of outside exploring a yard is to set up a pen in the yard with a couple EZ-up shelters over it (to block hawks and provide shade), and give some supervised exploring time. Some rabbits do really enjoy this because they can nibble the grass and lay in the sun, and if you are supervising it can be a safe activity.  If you want a permanent set up, you could build a predator-proof run in the yard, there are some ideas here: https://binkybunny.com/bunnyinfo/safe-outdoor-runs/

Do keep check whether you are in an area with RHDV2 though, and remember that going outside also increases the risk of picking up fleas, ticks, and other parasites.

My rabbits are harness trained but I don’t let them outside on it ( which was why I harnessed trined them ). There were too many hawks for it to be safe. I would worry with that backpack that he would chew the mesh. Like dananm said check on the RHDV2. A pen would be good but I think a stroller would be safer because there would be less of a chance of parasites. I know ibiyaiya makes a 4 i 1 and 5 in 1 rabbit carrier, the five in one is the stroller. Although it is pretty costly.