If you purchase a Garmin or Cycliq integrated camera/light, this can often replace any existing rear light you may have mounted on the seat post. If you wish to keep your existing light, could it be relocated to somewhere else, a seat stay, or perhaps the back of your saddle bag if there is suitable mount option.
Most action cams, have ‘folding fingers’ (2 tabs which slide between corresponding tabs on a mount - Gopro mount) and is relatively universal, resulting in a wide variety of mount options.
If you are fortunate enough to have space on your seat tube, there are dedicated seat tube mounts [1]. If you do not utilise a saddle bag, there are seat rail mounts [2]. Some saddles now include mounting holes on the underneath of the saddle which allow for third party mounts. These allow you to mount cameras, including GoPro, Cycliq, Garmin and others, as well as rear lights, and still support the use of a saddle bag (may require a short extension). A lot of these mounts are 3d printed, and can be found at Buplabs (US based) [3] and Concentric Cycling (Perth, WA) [4]
For those wishing to mount a camera on their handlebars, if you have enough space, then a standard handlebar mount would suffice [1]. Alternatively, some bicycle computer mounts, allow you to mount a device on the underside of the mount, Garmin [5]. If using Wahoo, then you would need to investigate 3rd party mounts [6]. Some 3rd party mounts can easily switch support for different computers through swapping the respective insert.
One option to consider is accessories from your bicycle manufacturer. For example, Trek have a range of ‘Blendr’ accessories [7] which may integrate with your stem and can provide up to 2 GoPro style mount points. Any GoPro mount can easily be extended, or split with the use of third party options. If you do purchase 3rd party accessories stay away from ABS plastic mounts as these can snap from constant vibration. Regardless of mount, strongly recommend tethering any device to avoid accidental loss - There is a GoPro somewhere in the countryside around Cowra.
Ultimately, there are many options, and what works well for one may not work for another rider due to individual preferences. If you want to add a camera, and are experiencing difficulty in establishing a suitable mounting point, I’ll be more than happy to explore options with you, specific to your setup.
[1] https://gopro.com/en/au/shop/mounts-accessories/handlebar-seatpost-pole-mount/AGTSM-001.html
[2] https://www.jbhifi.com.au/products/gopro-pro-seat-rail-mount
[3] https://buplabs.com/
[4] https://concentriccycling.com/
[5] https://www.garmin.com/en-AU/p/515324/pn/010-12256-22
[6] https://www.99bikes.com.au/k-edge-pro-combo-mount-wahoo-black
[7] https://www.trekbikes.com/au/en_AU/bontrager_blendr_how_to/
I have no affiliation with any of the links above - included for illustration purposes.