Lots of great points here. Regarding the astro lens: I'm not sure of any advantage of an RF lens in this category (stabilization and auto focus are not typically used). Sigma makes a great 14 mm f/1.8 lens with the EF mount, so all you need is the RF adapter.

I have the RF 100-300 and a 1.4 TC for it, and used it extensively on my recent trip to Brazil. My 500 f/4 hardly saw any use. However if a 200-500 f/4 entered the market I would definitely be tempted (especially if it had a built-in TC!).