I have now found out that Amethyst always crashes when I connect it via Orbot. It always works for a few minutes and then crashes. Without Orbot it works much longer without crashing. But I also suspect that it is due to too little RAM memory. That's a shame, because I really like Amtehyst! But unfortunately I don't have the money to buy a better smartphone. That makes me sad.
Discussion
You can run Amethyst clearnet through Tor if you use an app like Invizible Pro. I find that more relays work than using the proxy. You would need to figure out the settings to reduce memory use though as it does much more than Orbot currently.