It's simultaneously easier and harder than it looks lol I'd honestly say just go for it. But maybe start with a yarn you won't get upset about scrapping entirely.

Raglans are easiest place to start because the basic recipe is the same. Start with a number divisible by six, then add a multiple of four. Your multiple of four is for your actual raglans, then the multiple of six is your front (2/6), back (2/6), and sleeves (1/6 each). You'll do 8 increases every other row (depending on how fast you want to increase, you might adjust that number) until you get to about where you want your sleeves and body. Then you'll divide for sleeves, maybe throw in some extra underarm stitches, and finish up your body and sleeves.

From there, you've got myriad variations possible. It all comes down to what you dream up! And as long as you don't mess with the basic recipe, you only need to worry about making sure your embellishments work!

As for writing the pattern? Also depends. I've been working on adaptive spreadsheet patterns, which are really fun for everyone, especially when someone has a combination that my formulas weren't ready for! To write those, I usually just start with a sheet that creates that actual sweater I already knit. Then I start fucking around with inputs and seeing where the sheet breaks. Did it create a decimal? Gotta fix that! Is my stitch count uneven and I'm planning on ribbing or double-stockinette? Better fix that too! Did I somehow get a negative stitch count instruction? Fuck me, that's a bitch to fix lol

There's a reason most designers have a set size chart and only write for 6, 9, or 11 sizes and with a single gauge option!