It can be a lot of work to get Gelcoat looking really good and consistent. And it's such a bit investment of time!
Others have already given good advice here.
I just finished detailing the hull while the boat was out of the water.
I'll try to add some of my learnings that might help you.
Tools - You have a nice 5" Forced Rotation device. Be sure to use the right pad, it will make a big difference. When using compounds, you need a really firm pad or thinner, low-profile Mico-Fiber pad. Without the right pad, it won't matter how hard you work at it. You'll need to use a fair amount of pressure to make the 5" pad work. I have a similar polisher, and found it hard to cut enough to make the compound work the way it should. You might see if you can borrow/Rent a 7" orbital (Like the Makita 9227C as suggested here) with a wool pad (not foam or synthetic). I rented one for a day before buying and I can guarantee if used correctly you will see why the 7" orbital with a wool pad is the standard for pros. Swap or clean pads before then get caked or dirty as they become ineffective at that point.
Compound - There are so many choices and endless reviews and tests. if you talk to pros, so many of them use the 3M Compound (even after the recent formula change). It's in the same class as others (such as Menzerna) that are "Diminishing" in that as you use it, it breaks down so that the coarse abrasives become finer and finer as you work it, allowing a much finer finish than some other compounds. This is a huge benefit as it potentially allows you to get the work done with one less step. I've tried several over the years and keep coming back to these two products because of their ease of use and the level of finish you get after using them. Practical Sailor did a test on compounds and it shows there are a lot of differences between the products out there. Given the effort you will put in, this isn't the place to be pennywise.
Application - There are so many videos on this that I am sure you have seen. Key for me is to do smaller areas than you might want to do, work it more than you think, and be consistent starting with more pressure and reducing as the compound breaks down.
Evaluation - I used to compound, polish and wax, only to find that it looks pretty good, but not great. The level of work you need to put into different parts of the boat, decks, hull, port and starboard can be different given exposure and maintenance. To make sure you are successful in the end, take a look at your work as you finish each portion of your boat. I look at the reflection quality after finishing each section, getting up really close to see how sharp reflections are and how consistent the finish is. This is key as it will tell you if your work is yielding the results you want before going on. You can experiment with different pressure, products, pads, passes, etc for the area you are working. They make lights just for this, but I use my cell phone flashlight, hold it 3-4" away and run it across the area just worked on. If you look at the sharpness of the reflection, you will quickly know if you have done a good job or not.
If you aren't seeing a sharp detailed reflection, then try another pass.
Once you polish, protection is key. Unprotected Gelcoat, especially in areas with lots of sun will quickly become dull, stain and be hard to keep clean. Get a great wax. I really like the Collinite Paste wax. Just apply when it's cool and it will go well. if it gets hard to remove excess and get a shink just spray a tiny amount of distilled water on and then wipe.
To maintain the Wax finish, don't use any harsh cleaners or soaps (esp Dishsoap). as needed to keep it shiny, keep it that way by using a good two-step Polish/Wax such as Finesse-It II or Collinite 920. These products are perfect with your nice 5" Makita.
I hope that helps you a bit.
All the best to you in getting the results you want so you can get out and enjoy your boat on the water.
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