Grips are personal preference. The only universal rule is that you need to grip hard for long throws and softer for short throws.
I use the same grip for forehands of any distance and overheads. I use a 3 finger power grip for hard throws backhand and a pinch grip for softer backhand throws and putts. But my grips may not work for you. The issues are comfort, control and power. Experiment until you are satisfied and always be willing to try new ones. I still experiment, especially in casual rounds in terrible weather.
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The term "overstable" is relative. A disc which is overstable for you might not be for me (or visa versa). So a disc which hooks hard for you might go straight for me.
In my hands, I throw a Crush for an overstable driver (similar in stability to a Firebird) and a Flash for a straight, long distance shot (similar to a TeeBird). If I want the disc to go straight but less distance, I will shift to a less stable disc, like a Surge SS or for even less distance dial down to a Buzzz.
The problem which is unique to forehands is overcoming the flutter we generate. If the disc can't overcome flutter then it won't go straight or hyzer, it just flips over and never comes back. Some discs just don't fight flutter so they never earn a spot in my bag.
So you need to throw your discs enough to learn how they fly, how stable they are and how well they fight flutter. Eventually you will figure out how each disc flies when you release it flat at various speeds. And, yes, somewhere out there is a disc will will fly flat and straight at your normal power. You just need to find it and learn it.
How far can a good forehander throw a disc? Geoff Bennett thinks he might be able to get in the 600's in a distance competition. I wouldn't bet against him.
As you continue improving your form, technique and power you will throw throw farther every year for the next decade unless age or injury interferes.
You find yourself tanking shots because forehand has a smaller margin of error than backhand so missing by just a bit can give you a bad result. With practice and knowledge you will miss by less and less often. The hardest part of figure out is the winds (once you have solid form).
The most important thing for you to learn is to throw dead nuts flat. Gaining power is a secondary concern and will happen naturally with experience. Most forehanders do not throw flat which limits their potential.
It is useful to have a competent backhand and worth the effort to develop it. Just like a forehand, learn to throw it flat. Once you can throw flat any overstable disc will give you an easy hyzer. Eventually you will need every shot (overheads, rollers). Whatever you cannot do will bite you. So in practice and casual rounds try everything.
You will probably never become equally adept at both forehand and backhand (I only know of one golfer in the world who is) but happily the backhand is much easier to learn.
If you can get a video of yourself throwing I can give you more direct feedback. Don't worry about plateaus. They happen to everybody. Getting better means you are constantly making adjustments.
Keep practicing. Forehanders have to practice more than anyone else since what we do is harder than anyone else.