The only real reasons to go 300blk are either to suppress is or to shoot it in an AR.
If suppressing is not an option you may aswell go for 6.5 Grendel or one of the other cartridges you mentioned.
As far as barrel length, what ever you feel works for you.
18" is a nice mid length, for a bush only gun under less than 20" is probably recomended
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