As far as I am aware Skiff does offer the meeting scheduling with Jitsi and Proton supports adding an external URL but I don’t know of a service which has both and all the other features that you need. You may be able to request it as a feature though.
It seems to me that the best online calendar option is Skiff calendar which does have video conference scheduling plus it also automatically adds invitations to your calendar if you accept them. However it doesn’t support adding an external URL, you may be able to get in touch with them and they might add it. I don’t think a service that has all the features you need exists yet unfortunately so you might have to make a compromise here either way.