The hybrid fuel grain need not be plastic, and can be cast in complex geometries or even 3D printed in such. There are no real issues scaling the grain up to whatever size you want.
There are issues scaling the grain if the material is such that it does not scale well, such as a wax. At about 6", grains composed primarily of wax tend to suffer slump failure during burns.
Practical is sort of a cost/benefit equation. Since AP has way more than doubled in price these last few years the financial appeal of hybrids increases. Hybrids are MUCH cheaper to launch per newton second.
The real problem is most clubs got rid of, or mothballed, their GSE. GSE for hybrids is additional hardware that is not needed for solids. This can be a fill station of sorts - for pre-fillable hybrids, or solinoids and control box etc for those that fill at the pad. In all cases a nitrous source tank is required. For hobby motors, the oxidizer is required to be N2O.
At the moment, I think hybrid GSE doesn't cost more than one decent sized solid propellant reload, say, for a L3 cert flight. Prices have changed.
It's a chicken and egg thing. Clubs don't set up GSE at the fields. Manufacturers aren't making much in the way of kits for hybrid motors since they aren't selling many. Not many would make a hybrid rocket when their club doesn't provide any support. Let's face it, HTPB solids are easier to deal with at the field. But they definitely cost a whole lot more in the long run. That cost is going to push people out of this hobby and provide more of a barrier of entry for others.
If you don't know, CG shift during the burn is a bit different for hybrids than for solids. Hybrids also tend to have a greater L/D (length to diameter ratio). These things are the differences that need to be taken into account when making a rocket for a hybrid motor.
Sugar motors, and N2O hybrid motors, are pretty cheap to fly on a per-flight basis. HTPB, not so much so.
Picture is of an EX hybrid motor I made, mock assembled, that I hope to fly some day. I have my GSE, just no nitrous supply tank and nitrous. And COVID has put a crimp on many things.
Gerald