The biggest mistake people make is clamping their mower blade in the vise and grinding to a nice sharp edge with a hand held angle grinder. This is actually the worst thing you can do.
This newly sharpened blade will immediately lose its edge in use and rapidly get duller.
Blades require a “hollow” or concave profile to maintain a proper cutting edge. No matter how steady you think you are you are in effect creating a CONVEX profile without the use of a jig.
Hard to believe that you're doubly backwards on this.
1) Convex blade profiles take longer to lose their edge and get duller much slower. Problem is, they were never as sharp (well cutting) in the first place.
2) A grinder with a round wheel inherently creates a concave profile. You're doing something terribly wrong if a metal grinding wheel makes a convex profile against a blade edge, possibly trying to take off too little material at a time and too many passes at different angles?
At the same time you don't want it too concave or it'll get dings too easily as well as rapid dulling. The slightly concave, near flat grind from an angle grinder wheel works well for me, a reasonable trade off between new cutting performance, ding resistance, and dulling. It has worked well on my present blades which I've used over 20 years (along with a 2nd set to swap in, already sharpened, when I service the deck).
If you need a jig, you need one. I don't. No blade sharpness problem. You're just lacking proper technique. I'd imagine there are youtube videos out there but if you're already invested in a flap wheel and jig and get acceptable results, you might as well
stick with what you know... just as I will.