I picked up the Canon RF 100-400mm f/5.6-8 IS USM a few months back specifically because I was tired of lugging around heavy EF glass with the adapter. It really is the main choice for that budget, but let me tell you, it actually handles the 32.5MP sensor on the R7 better than most people give it credit for. I took it out to a local wildlife refuge last weekend and the autofocus tracking is night and day compared to the older EF 70-300mm. It is tiny too... literally fits in a small shoulder bag. If you're worried about reach for those shorebirds, you might even look at the Canon RF 600mm f/11 IS STM. I found one used for under $600 and while that f/11 aperture sounds scary on paper, the R7 autofocus still sticks like glue in daylight. I was worried about the high pixel density making everything look soft, but as long as your shutter speed is high enough, the images are plenty crisp. Honestly, if you can find a refurbished deal on the Canon site, you might even be able to grab both if you're lucky, but the 100-400 is definitely the most versatile for a trip where you dont know how close you will get. Just make sure you have decent light because once the sun starts dipping, that f/8 or f/11 starts to show its limits with noise.

Been looking at the data for a while because that R7 sensor is basically a torture test for lenses. With a pixel pitch of 3.2 microns, you're hitting the diffraction limited aperture (DLA) at roughly f/5.2. This means anything you shoot at f/8 or f/11 is technically already losing some fine detail to physics, but the native processing handles it way better than the old EF stuff. The Canon RF 100-400mm f/5.6-8 IS USM is the logical choice for $650. It uses Nano USM which is significantly faster than the micro-motor or ring-USM in older budget lenses. It covers about 160-640mm equivalent on your body. If you strictly need reach for shorebirds that wont let you get close, the Canon RF 600mm f/11 IS STM is right at your $800 limit. It uses diffractive optics (DO) to stay light, though the fixed f/11 means you need a lot of light to keep the shutter speed high enough for birds in flight. If you dont mind second-hand, you could look for a Canon EF 400mm f/5.6L USM. Its an older design without IS, but the optical quality is still top tier and it focuses incredibly fast. You'd still need the adapter, but the purple fringing is much better controlled than the 70-300mm you're using now. personally id go native tho... the integration with the R7 autofocus system is just way smoother for tracking fast subjects. The 100-400 is basically the sweet spot for that budget.

• Stick to native Canon glass.
• I struggled with adapted gear at the coast for months, but native mirrorless glass works way better for tracking fast shorebirds.