"...The D500 would replace my beloved D300, which I do not use to shoot games; but otherwise works well..." lesdmd there is indeed a very elegant and cost effective solution to your desire for high frame rate...
And in spite of the accolades which surround the D500 here is another thought on the highly rated D500.... This is the only camera I returned to B&H after shooting two league soccer games in a weekend... Why? Image Quality and want of dynamic range... yes the flagship DX DSLR D500 has lots of compelling features but dynamic range isn't one of them in my humble estimation... And those in denial who claim there is virtually no difference between it's sensor's 20.9 megapixel resolution and that of the D7200 sensor's is 24.72-megapixel resolution may need to rethink this... however they still insist to extolling it's virtues....
However DxOMark believes otherwise... and they have been testing sensor for quite some time... btw their findings echo mine...
https://www.dxomark.com/Cameras/DxOMark rates the D7200 as virtually equal to the Nikon D5 in sensor score with an over all of 87 verses 88 while the D500 is far behind at 84... even more telling is dynamic range score (most important for IQ) with the D500 clocking in at 14.0 again far below the D7200 at 14.6. Nope if you need a camera that is equally capable of shooting portraiture, fashion/beauty narratives the D500 may not be a wise option.
lesdmd my best advice to you is to pick up a used MB-10 battery grip and with a Nikon EN-EL4a battery... this combo on your D300 will give you 8fps no problem... albeit you'll need to switch to 12bit RAW to realize this... there are lots of MB-10's available (cheap) on eBay or if you prefer a slightly higher price from KEH, B&H etc.
This combo works extremely well (I have this piece of kit) and in bright daylight the D300 yields very good results indeed... The only issue is with the D300 is it's rather serious limit on high ISO performance... You'll want to set Auto ISO to top out at 1600 max... which even for a 5.6 aperture is totally doable... ISO 400 will give you 1/2000 second in bright daylight at even f/8!
If you do find that the high frame rate is where you want to be, then you might consider the D500 and join the spray and pray crowd... lol yep, you can fill memory cards very quickly with the D500.
btw, I rarely shoot sports now... the revenue stream simply isn't there... too much effort for too little recompense... but shooting for your grandson's soccer games can be an often is virtually priceless...
Hope this helps lesdmd or is at least food for thought...
And while I'm a tad late to this dance I realize there are likely many other D300 owners who may ponder over the high price tad on the D500...
I wish you all well on your journey...