Sorry I didn't mention: it is a web game. Accessibility without JS is not possible.
In that case, JS-only is fine, but presumably you're talking about serving fallback content (bots/crawlers are unlikely to be playing your game)
Yea, I will try some better fillers than just "loading...", and the metatags stuff.
I also might consider serving some other non-game pages statically, like the leaderboard, user profiles, etc.
Rewrite it to do server side rendering.
Sorry if this seems facetious, but if you want it to be accessible, it needs to be server-side rendered.
Some crawlers have some (limited) JS-rendering abilities; notably Googlebot; but it's not full. Some crawlers may use a fully-fledged Chrome-headless but latency would make this uncommon.
In general, JS-dependent SPAs have always been a terrible terrible idea for anything public facing that needs to be crawled (they're fine for web applications, not for web pages).