Minecraft wiki entry for Starting_in_the_Nether, associated text: If you build the farm at the top of the nether (Y=120), you can just have the mobs drop off and die from fall damage, and then run around below them picking up the drops.
The list of air blocks that mobs think they can pathfind over tends to change over time, it seems, so make sure the method you're using actually works the way you expect. They still drop off without any fake blocks, it just might take longer.
Your drop platform should be made of blackstone bottom slabs. That way ghasts can't blow it up and piglins and hoglins won't spawn on it.
If you spawn proof with lava, then striders spawn, ruining the rates (I tried it), so find a place with enough area that you can actually stop things from spawning if you stand at the right place. Build a standard mob farm with the AFK spot near the roof. You'll need to find the sweet spot between being high enough above the lava lake, and low enough the mobs don't despawn as they drop. You can also spawn proof with half slabs if you want.