Expression to introduce an immodest statement

Well you, yourself suggest, 'Modesty aside'. It is quite commonly used.

Google ngram: modesty aside

Example

Of course, all modesty aside, I'm the better swordsman. The Phantom's Opera By Sadie Montgomery 2007

A frequently used idiom is:

"I don't want/like to blow my own trumpet but ..."

Example: I don't like to blow my own trumpet but I think you will find I'm rather good at answering these questions.

Here are some more examples

In UK English a common phrase used for this purpose is "though I shouldn't say so myself", for example:

Though I shouldn't say so myself, I am an excellent darts player.

There are regional variations of this, such as "though I say it as shouldn't" which can be rather impenetrable, but the standard version should be clear to any English speaker.