I had to buy my Order cleric from Ravnica in D&D Beyond and its now level 8 and i am wanting to use the optional Blessed Strikes from my Tasha's hardcover.

The optional class features are not a stand alone item that i can buy. are they part of "the compendium" ? do i have to purchase all of that to get the optional features on my character sheet or do i have to go further and re-buy the whole book in digital form? it seems odd that the optional class features are not part of the "a la carte" options https://mobdro.bio/ https://kodi.bio/.

its really unclear what "the compendium" section contains and does.

or would it be simpler to just use D&DBeyond's "Homebrew" to set it up from my hardcover - i have tried several ways and none are very satisfactory.

You have to purchase the whole book on this platform to unlock optional features, unfortunately. If you don't want to do that it's homebrew, yeah. I really do wish it was clearer.

You actually cannot even homebrew some of them. Anything that replaces a base class feature is un-brewable. 

Well, you can add the feature, you just can't make it replace the old feature, so you have to remember you're not technically supposed to have the old feature (revised ranger folks have been doing that for ages before Tasha's, so it depends on your tolerance and which features have the ability to edit at least the name in the sheet to add a 'don't use' note).

Tasha's Cauldron of Everything, the book those mechanics were published in.