This questions has probably been asked many times;

but I want players at character creation to have access to my own homebrew (and not all the homebrew of everyone else); is this possible?

As far as I know, the allow home brew slider on the Home tab of the builder doesn’t discriminate between yours and others home brew. 

Maybe you can make your home brew classes as characters, level one, in your campaign unassigned and have your players select those to use. Then they can build them from there?  I haven’t done this myself so I’m assuming when a player takes an unassigned character they can use the builder to make changes to race etc.