Get your slimy, webbed phalanges off ma boots!
When drawing the initial hand of 10 cards, I am discarding and replacing any enemy breakthrough or ultimate weapons cards.
Is this the proper way to play or have I reduced the dificulty when doing that?
Nothing saying you can't continue to play that way, but yes, it would be a house rule. In the official rules, there is nothing to indicate that your opening draw is treated any differently than any other draw.
That means you should discard ultimate weapons cards without replacing them (since you clearly don't have the ultimate weapon upgrade on turn 1), and any drawn enemy breakthrough card should be resolved and not replaced.