An option not mentioned is a Carradice Bagman QR & traditional saddlebag.
This fits to the saddle rails, which should be steel, with enough rail free behind the saddle clamp that you could move the saddle forwards about 15 mm.
Various sizes of saddlebag available, between about 7 L and 24 L. Bag removal is a couple of seconds, 5-10 to refit, bagman frame removal/refit is a couple of minutes with an allen key.
Not cheap, and you may not like the retro image.
The problems with luggage carried on the person are sweatiness, and what happens if what you are carrying ends up between you and the road when you crash.
The latter may be just padding making for a comfier impact, it may be a hole in the leg from the bunch of keys in your pocket, or it could be something hard and unyielding leading to a broken back. Just don't believe yourself if you think you'll never carry anything that could be a problem.
In the end, you pay your money and make your choice.