How to make a good tasting pepper cream sauce without using premade fabricated stuff
I haven't made peppercorn sauce for many years, but from recollection, how you combine the pepper and the fat is the key.  Many sauces use cream, but it's not completely necessary.  The pepper flavour is staying in the peppercorns, which is why it doesn't come on early.
I suspect you need to cook the pepper in the butter before making the roux.  This will extract the flavour into the sauce better, as piperine (the most important component of the flavour) isn't really soluble in water.
Some recipes use alcohol, which also dissolves piperine, e.g. this one (in French) makes a reduction using cognac and adds the butter later.
White pepper is also quite common; I can't instantly find a recipe but some use ground white pepper in a cream-based sauce, with whole black or green peppercorns.  If you're using ground white pepper, buy it fresh; it doesn't keep its flavour very well.
As for the  saltiness, are you adding as much as the commercial sauce? Commercial sauces often use shocking amounts of salt, but you might not need as much once the pepper flavour is dealt with.