French press as stated and aeropress will give you the most consistency across grind sizes. But a burr grinder is the most important piece of equipment for coffee. I was sceptical uuntil i got one. Surely the difference isn’t that big? Boy was I wrong.

Consider a hand grinder, even something cheap like a Harrio annoying as it is to use, it is passable at like 20 bucks. It was my first grinder, it did the job, but adjusting the settings is limited and annoying and it took forever to grind a cup, I quickly bought a Mignon Chrono.

If this isn’t an option, then maybe consider buying preground in smaller batches from a local place? The place I order my beans from offers to send them ground in a couple of generic preset sizes.

But the problem is not only the quality of supermarket coffee, and the preset grind size, it is also that ground beans will oxidise much more quickly and even when vacuum sealed, the moment you open the bag it will start to deteriorate and quickly. I would possibly even consider a blade grinder over buying pre-ground coffee, especially supermarket coffee.