It all really depends on how much you are comfortable spending at this point.
Unless it isn't in the budget to go forward with mirrorless, I wouldn't make huge investments in DSLRs. I don't think any manufacturer is supporting DSLRs long term and are moving everything over to mirrorless.
That said, I am in the process of moving most (if not all) my stuff over to Nikon mirrorless, so will probably be posting my DSLR stuff (at least most of it, probably not moving my 400mm 2.8 or 200mm f2.0 just yet) for sale pretty soon.
There is a EF to FTZ adapter for the mirrorless cameras you can buy so you can use the older lenses with your newer mirrorless cameras.
I need to send my D500 into Nikon, but the rest of my stuff is in good working order, but probably won't be the prettiest gear you ever see.
Bodies to sell:
D800 (~125k shutter clicks) $400
D4 (~700-800k shutter clicks) $500
D500 (need to send to Nikon first, so price unknown)
Z6 (I have the Z9 and want to upgrade this, have box and everything) $1000
Lenses to sell:
Nikon 14-24 2.8 $750
Nikon 24-120 f4 $350
Nikon 28-300 3.5-5.6 $500
Nikon 70-200 f2.8 VR2 $800
Nikon 105 2.8 Micro $400
Nikon 35mm 1.4G $650
Nikon 85mm 1.5G $850
Nikon 50mm 1.8G $100
So depressing to see what these sell for now, compared to what I paid. The D4 was like $7500 or 8k when I bought it. Pretty sure I still have all the boxes and everything for all this, except maybe the 28-300 and 50 1.8.
I still have some Canon stuff as well, but I need to go through that again.
D4 would be your best value camera possible for the HS sports. The sensor is fantastic and my D4 bodies are what I used this year for all my XFL pictures for the XFL.
But yea, I love my Z9 so much, I am ready to move it all over.