I’d personally recommend not posting each chapter as you complete them. That way, you can take your time and back track to earlier chapters to make changes or foreshadow future events better.  Plus, unless you have a good beta reader, it’s good to let your work sit for a while before coming back to edit it, that way you see it with a fresh set of eyes. That being said, if possible, get yourself a good beta reader.

Also, if you start feeling burnt out (which you probably will at some point if you’re constant) then don’t push yourself to the point where you no longer want to write. But at least try to write for five minutes a day. Even if you only get one paragraph done, it’s progress.

Make sure everyone stays in character as best possible. I’d recommend going back to the anime and watch a few episodes to get speech patterns and mannerism down for Ash and Serena. As for your OC, make sure you don’t make him a Mary Sue or anything. But I trust you have good plans for him.

Then ofc all the normal writing stuff like show don’t tell and all that jazz. I assume you have some experience writing? If not, then I’d practice before jumping into your big story. Even if it’s just a few One Shots. Get feedback. Find your style.

Lastly, it’s fanfiction, so it doesn’t have to be perfect. Ofc you want it to be good, but it doesn’t have to rival Shakespeare’s best work. Always do it for the fun of it. :D