The new "auto" mode should be the default for anything, also in TinyMCE.
I'm going to work on the relateditems default pattern settings (setting the start path to the current context by default, as this was the default in Plone 4.x too). It's probably a good idea to align the related items widget settings for TinyMCE with the generic settings in plone.app.widgets.
Besides: I'm not very happy with the TinyMCE settings adapter too and the need for plone.app.widgets in general. Probably should be simplified at some point.