The Oly 100-400 is a pretty big investment, but its actually very good overall. And if you already have the 1.4 T/C it works on that too.
I've always wanted to do some serious astro/ planetary stuff with a decent telescope but I've never had the funds/knowledge to take the plunge. I think it'd be lots of fun to get shots of Mars, or Saturns rings, or close detail of the moon etc. But its quite an investment by all accounts.
The issue some of us have is "light pollution." The reflection of the city lights by the particles in the air.
That can really degrade the ability to "see" astronomical stuff. And it reduces the contrast of whatever you are looking at.
The research is enlightening.
But like many things in the internet world, there is a LOT more info than there was in the past, so you can make better decisions when planning what to get.
And that helps to make the compromise decisions you have to make.