I have a Cygolite 460 that I've been using on my road bike for daytime riding. It's got a bright flash daytime mode and will last for longer than I am ever likely to ride in a day.

I just recently purchased a Garmin 530 and its pretty crowded on the bars now. My Roubaix has only a small area near the stem where the bars are round. The cygolite only has mounting provision for round bars with a rubber strap and can't attach to something that would fit the underside of the garmin mount.

Anyone have suggestions on a light that will easily adapt to the garmin mount that has a bright daytime mode that will last 8 hours or more on a charge? I almost never ride my road bike at night so daytime mode is my primary concern.

Mount it upside down, underneath the bars?

Otherwise, get a combo Garmin+GoPro mount, and then there are several options you can check out here: Bright front light that mounts on aero road bars? - Bike Forums

Lezyne has some lights that can be mounted hanging (the call those versions "Loaded" or "reverse"). Otherwise there are adapters like this (german shop, but also on aliexpress and the like available), which allows any light to be mounted, if there is a downward facing GoPro mount.

If money is not a limiting factor: outbound lighting focal road in conjunction with a outfrount GoPro mount.

Have you considered a handlebar extender? It basically lets to hang a "round" bar below the bars, and you could mount your light to that. They used to be the thing many road cyclists used to mount a light under their computer before the advent of GoPro mounts.

https://www.amazon.com/UPANBIKE-Hand...%2C309&sr=8-25

If you like your cygolite, buy a cygolite helmet mount and take it apart. The part the light mounts to is GoPro mount compatible. Use that with an OTF mount like a k-edge or a knockoff with the Garmin up top and the light on the bottom.

One thing to look it for is that the OTF mount you buy is long enough for the light to clear your cables.

Not sure how the strap on the 460 is mounted but this adapter may work. Then you need a Garmin out front mount and a GoPro mount. This is what I did but with my Trek light. The parts add up though.

https://www.shapeways.com/product/5J455WDWZ/cygolite-dash-to-gopro-mount

Computer not mounted.

Depending on the number of spacers you run, you might have enough room to mount a light sideways on the fork tube under your stem.

Um... the Garmin light with Garmin mount dual sided mount? Am I missing something?
You can also control the light from the head unit if you’re content to live on the Garmin ecosystem.
Also, check out the various K-Edge Mount solutions as well.

minoura swg-400 stem handlebar accessory mount holder

Late reply, but not a completely necro thread:
Since you have the Edge 530, this mount will convert any GoPro style accessory (light or camera) so it can be mounted on the bottom of the out front mount that comes with the non-mountain bike version of the Edge 530.

It's marginally nicer than a mount with just a GoPro adapter on the underside because it's easier to remove the light or camera, but it's still very secure. The drawback is the Garmin out front mount is plastic, so it may flex and vibrate more with a lot of weight on it.