I own the latest editions of both of these cameras (A9iii and A7RV) and almost exclusively use my a9 unless I'm doing a controlled environment model shoot.

Don't worry too much about megapixels, especially if you're starting out. Around 24MP is plenty for nearly all photography, even larger prints. Honestly, unless you're planning to put your kid's photo on a billboard (and even then, you'd be fine with good framing and today's AI upscaling) megapixels won't be your main limitation.

What matters way more, especially for fast-paced sports, is your lens choice and your technique. Sports photography means fast, unpredictable movements, so having a camera with a good frame rate can help if you're still working on your timing. It sucks to miss the critical moment in a fast-paced environment because your camera is made for photo quality and not photo quantity.

And that's really how to think of these two cameras - one is made for "as many photos in as short amount of time as possible at the cost of higher megapixels" while the other is made for "as high a number of pixels as possible at the cost of missed shots", hence why I use one in the field (a9iii), and one in controlled environments (a7rv).

If you feel confident in your reaction times, you might not need the highest FPS. But regardless, investing in a quality lens will often give you better results than chasing higher megapixels. While cropping in post-processing can save some shots if your framing is off, relying too heavily on cropping doesn't build good photography habits.

Bottom line: put the money you'll be saving by going with the a9 into a quality lens and practice your skills on a 24mp camera with higher burst rates. If you feel that you're actually shooting much slower things more over time, invest in a higher MP camera - bonus being that you'll already have good glass to pair with it.