Whatever the project it needs to be something you'll passionately work on for twice as long as you think it will take. That last 20% is dangerous territory!
Yes, the final strech is extremely daunting. For some reason it haunts you with doubts and it feels like there's no light ahead. I'm there currently but I think I handle it quite well for now. But it's a grind regardless.
releasing the game for platforms I don't personally have access to since I don't have a good way to troubleshoot the problems.
You can set up a virtual box with a linux and a mac install. It's fairly straightforward with a tutorial, just finding a mac iso is not that easy. Doesn't help with steamdeck though.
IMHO this should be the most important thing in your roadmap, but it is just my opinion it's up to you.
From what I gathered a very important part is apparently translation. So covering the big languages English, French, Italian, German, Spanish followed by Simplified Chinese, Japanese, Korean followed by Brazilian-Portugese, Russian and Turkish. I'd argue most linux users are savvy enough to run wine anyway, and doesn't the steam deck do that too anyway?