For a road bike, a belly band and lightweight gun would be the answer. Or wear a sturdy belt with OWB holster directly on skin over bibs but under jersey.
For a Mountain Bike: I like to wear a hydration pack and use a Safepacker secured to the belt and strap. It is subtle, secure, safe, and slow to draw. But not as slow as you might think.
I am wearing the Safepacker with G17 in this video.
The terrain is difficult and it carries well. A G26 carries even better and is hard to see. Visible in this photo:
I can’t AIWB on any bike because my balls get crushed. Some people find a way to do this.