Out of your camera choices, the R6 Mark II is going to be your best bet because of the generally better performance of its full frame sensor in low lighting so typical for sports and unlike the other full frame (R8) in your possibles list the R6 Mark II has a larger battery which you will need for sports. With either of those bodies, you will need to bring batteries but with the R8 you will need several batteries for an event.
Also when you get serious about shooting sports, make sure to keep an eye on the battery and change it before it shuts the camera down in the middle of a good play. Lens AF performance will also begin to drop as the battery reaches critical discharge and this becomes particularly noticeable when shooting a burst during a fast moving play. I am not moving away from my 1DX III bodies anytime soon and battery life is one reason since they will do around 5,000 to 7,000 captures before it is due for a recharge so I typically make it through 2 or 3 events between charges and don't worry about needing to change batteries during an event. It isn't a big deal but you do want to be sure to make the battery change at an appropriate stop in action rather than miss something critical.
With sports, just because you can acquire a huge number of images at close to zero cost doesn't mean it is a good idea. The "spray and pray" approach to sports photography often results in a ton of mediocre images and one or two usable while using a more skilled approach forces you to stay highly involved with the activity which will yield better shots and far less to do in post. Last Friday was a typical HS football night for me, I captured just under 1,000 images between two bodies and kept 270 of them. Well over half of the images were pretty good but after two or three great shots of a particular running back cutting through traffic you don't need 50 more 😂
Location is critical to sports photography and you have to know the sport well enough to know where the play is going a high percentage of the time. You also have to be smart about not having your view blocked, for example when shooting from behind the end zone you want to make sure that the goalpost doesn't come between you and the point of reception or the point where a runner crosses the goal line. Of course you have to stay well out of the way of officials but you also have to be aware of their location and its impact on your photo. An official signaling a TD in the background of the player crossing the goal line makes an excellent photo but an official signaling a TD who is standing between you and the player is no good. Just a couple of steps to one side or the other will often be the difference between a great photo and one destined for the digital dumpster.
Your 70-200 f2.8 is a great sports len, I always have an EF 70-200 f2.8 on one body. I have never used the Tamron 24-70 f2.8 but it should also be a good lens and is excellent for shooting low and close during basketball. You will want a longer telephoto for football and it needs to be fast and that unfortunately is expensive. I used a Canon EF 300 f2.8 for a couple of years for football and I think it is probably optically the best lens ever made by Canon and produces beautiful images; I now primarily use a 400 f2.8 which has a little better reach and the optical quality is also excellent but I don't think it quite matches the incredible EF 300. Unfortunately all of the long fast primes are expensive but a used one should be a serious consideration.
I think you will find your variable aperture 75-300 too slow in both maximum aperture and focus speed to be a good sports lens except for daytime sports. The 70-200 f2.8 and a fast longer focal length (primes are GREAT) will make a great long-term combo for you. The first two photos below were captured with EF 400 f2.8 glass and the second two with the EF 70-200 f2.8, all on 1DX III bodies.
Rodger
