It sounds like you know what you're talking about until one realizes the earth is spinning. Wide field photos can be shot up to thirty seconds depending on the back and lens.

Anything more zoomy than 50mm uncropped you're getting streaks in < dozen seconds. There's a rule of thumb but I don't remember it.

Best course of action is to take a video and let a stacking program deal with it, especially if you use a real telescope.

Also the Sony a7r have like "150,000 ISO " and iirc cost like $3500 with a kit lens. That's a bit above consumer, but I may have mixed up models.

My camera is not an a7r. I paid $1200 for it used.

The exposure was 10 seconds, so by your own explanation, the spinning of the earth is not a problem ( as you can clearly see in the photo I linked )

Rule of 500. Divide your lens focal length into 500. That's the longest exposure to avoid trails. 500 / 24mm = ~21s so your example would be 500 / 50mm = 10s.

Some random search result:

https://astrobackyard.com/the-500-rule/

Tens of seconds is about right. It's something like 500/f(mm) in seconds, but you get a feel for what will blur and what won't. Here's an example I shot with a Nikkor 50mm f/1.4 @ 20s: https://www.instagram.com/p/C-mU6iIp0re/?igsh=cm16bWx1cGp3OG...