Like someone mentioned, that 1.6x crop factor on your R7 is a total game changer for wildlife! It effectively gives you a massive boost in reach without the added weight of full-frame super-teles. Since you are on a strict $1500 budget but want fantastic results for Yellowstone, I have a few technical paths that are actually super effective:

• Look into the Tamron 150-600mm f/5-6.3 Di VC USD G2 for Canon EF. It is currently priced around $1,399. The image stabilization is rated for 4.5 stops and the G2 autofocus is incredibly snappy on the R series bodies. You get a massive 960mm equivalent reach on that sensor!
• If you want to stay native and keep the weight down, the Canon RF 800mm f/11 IS STM is a total steal at $999. It uses gapless dual-layer diffractive optics which makes it light and sharp despite the fixed aperture. On an R7, that is a 1280mm equivalent field of view. It is honestly unmatched for long-range birding or wolves across a valley at this price.
• Another fantastic move is just renting the Canon RF 100-500mm f/4.5-7.1 L IS USM for the week. It costs way less than buying it and lets you use the gold standard glass for your trip. Spend your extra cash on a high-speed card like the SanDisk Extreme PRO 128GB UHS-II SDXC so you dont hit the buffer during high-speed bursts. Have an amazing trip, Yellowstone is stunning!

The Canon RF 100-400mm f/5.6-8 IS USM is actually a solid, safe bet for your budget. It works fine. Honestly dont worry too much about f/8 if you just bump your ISO for those early dawn shots. If you want something more robust, the Canon EF 100-400mm f/4.5-5.6L IS II USM with the Canon Mount Adapter EF-EOS R is super reliable and worth the weight. Let me know if you need more help.

Building on the earlier suggestion, honestly dont sweat the f/8 issue too much on the R7. I spent a whole season in the Tetons with the Canon RF 100-400mm f/5.6-8 IS USM and it handled just fine. People get hung up on the aperture numbers, but modern sensors handle higher ISOs like champs. If you really want to keep your budget in check and still get that extra reach, you could look at the Sigma 60-600mm f/4.5-6.3 DG OS HSM Sports for EF mount if you can find a used one. It is a beast to carry around compared to the native glass, but the versatility is insane for Yellowstone where you might have a grizzly a mile away one minute and a bison walking right by your car the next. Just keep in mind that with any EF lens, you gotta factor in the weight of the adapter. Native RF glass is definitely lighter, which matters when you are hiking out to spots. Whatever you pick, just go shoot. Yellowstone is gonna be awesome regardless of the gear.

Just saw this and thought i would weigh in. Reach is usually more important than aperture for wildlife if you are shooting during the day, especially since the R7 sensor has a 1.6x crop factor. Since the 100-500 is out of budget, here are a few other technical routes:

• Sigma 150-600mm f/5-6.3 DG OS HSM Contemporary for Canon EF. This is a classic choice. You get way more reach than the 400mm, and it stays within your budget even with the adapter cost. Just a heads up, some people report focus pulsing in servo mode on the R7 with this lens, but it is usually manageable.
• Canon RF 600mm f/11 IS STM. It sounds crazy to go to f/11 if you are worried about low light, but this lens is incredibly light for hiking around Lamar Valley. The R7 can handle higher ISOs better than you think if you use some denoise software after.
• Tamron SP 150-600mm f/5-6.3 Di VC USD G2 for Canon EF. Pretty similar to the Sigma but has slightly better weather sealing which might be nice if it rains or gets dusty.

Noted!

Works great for me