my thought is that i have to hook on epi-link somehow.. 

Website\modules\_protected\EPiServer.Cms.TinyMce\EPiServer.Cms.TinyMce.zip\2.6.3\ClientResources\epi-addon-tinymce\plugins\epi-link

or maybe we can add something in the TinyMCEInitializer... example,  .AddSettingsTransform but im not quite sure what to do.. 

I would put it in one of the server side events (OnContentSaving or similiar) or if you already have masses of pages; when rendering.

@Goaral: which version of EPiServer TinyMCE are you using? I think you should not get that issue from EPiServer TinyMCE 2.0

Had to do something similar a while ago, although in our case we couldn't keep it specific to the Episerver/TinyMCE rendering as the client had hardcoded links in their FE that they also wanted all updated in one sweep (and not having to worry about it in the future is a bonus).

First I did it with an Attribute Filter on the base page controller which worked, but the way they have custom cookies meant it didn't work that well with A/B Testing, so v2 of the implementation uses an HTTP Module.

You can find the full info here (as well as code samples): https://jcpretorius.com/post/2018/replace-all-occurrences-of-something-in-the-http-response

Jacob.