Q: How to prevent accidently update to 5.800?

I’ve successfuly downgraded to 5.600
Now I don’t want playstore to accidently upgrade to 5.800.

I’ve already unchecked the “auto-update” feature, but I think that “update all” will also include k9?

You may be able to accomplish the same intent with google play store settings. With version 26.3.16-21 you can turn on auto-update globally and then turn it off on an app-specific basis.

Once auto-update is turned on globally, select an app, and then the three vertical dots in the upper-right. The “enable auto update” box should be checked - uncheck it.

I have not tested that this really works as implied. Also, if app updates are pending you have to be careful in your handling of “update all”.