If you go monopod, you don't need anything hugely expensive, other than a sturdy tilt head for the monopod. Many sources. In US, Custom Brackets (the one I have, excellent quality, lacks a feature seen in newer designs), Kirk, RRS, Hejnarphoto machine shop, plus you might be able to get direct imports of Chinese and Taiwanese knockoffs.
I know that local people use and like the Jobo Junior 3 sidemount gimbal (lightweight and simple) for the EF 500 f/4L IS v.1, at ~3.9 kg for the lens. The action is good with this lens - I tried moving the gimbal/lens at one of the local bird photo hotspots. Photographers check out birds, then when things are slow they check out other people' kit.
If you want to use it for other lenses where you may be using the camera L bracket as clamp point, look for those heads that allow you to switch orientations in the field, generally requiring a hex key/ allen wrench (or whatever it is called in Europe). I have one of the older sturdy tilt heads without that feature, and I rather regret it, as sometimes I can use a monopod where tripods aren't allowed, don't really fit, or are otherwise hard to set up. Monopod plus image stabilisation is better than nothing.
