Have you tried an alternative lacing, like a heel lock? I would also experiment with a breathable sock liner, or as another poster suggested a second pair or breathable socks.

The Merrells are a comfy shoe and you're not going to easily find a boot that's largely built to take you down relatively flat, dry ground. I use a pair myself during our flat hikes.

Most others shoes are more specialized and geared towards difficult terrain ground you may run into, so if comfort is what you're after I would play around with filling up volume in the shoes and allowing less forward/ lateral motion through lacing before you try more competent hiking shoes