300k, wow. If it's still working and you're happy with the image quality etc, then why change? Don't fix what isn't broken. The 7Dii was discontinued a while back now. There will apparently be no 7Diii, more likely, there will be a crop mirrorless R to replace the 7D line. (R7?)
That said, depending on budget, if you're thinking of upgrading, then mirrorless is the way. I'm quite happy with my current DSLR but when the time comes, I will be going Canon R mirrorless.
I'm presuming you're bought into the Canon lens system, so you'd be looking at an adapter too to switch to mirrorless. Also presuming your budget is about £1.5 k, going on what a new 7Dii would've been.
You'd be looking at an R or RP for that money, the newer Rs are considerably more. The R has reasonable performance, even by today's standards although it is slightly older tech. If memory serves me right, the R has the same sensor as the 5Div and I can vouch for that being a lovely sensor.
It would probably be less faff and no need for an adapter to stay with a DSLR but they are gradually being phased out. If you don't mind that though and for the presumed £1.5k budget, you wouldn't go far wrong picking up a decent used Canon full frame body, you would probably get a 5Div used, with a few clicks on the clock for that money. I don't know what lenses you have but any EF fit lens will work on a Canon FF body.
I also have an M5, not had it long and it has really suprised me. The M6mkii is the latest, crop sensor mirrorless from Canon but you'd be looking at an adapter again to use your current Canon glass. I'm not sure the M series is entirely suited to sports though.
It's all a bit subjective and it depends what you want to do, horses for courses and all that but that would be my advice, based on the assumption you already have Canon glass.
There's the spectre of a brand change too, but be prepared to spend tons more on lenses, which is why I have suggested a Canon body.