Yikes, that crank arm has seen better days. Given your situation and your comment that your drivetrain is entirely worn, I would suggest that you clean the joint as best you can (so that dirt isn’t getting in the way of what remains of the splines), tighten the crap out of the pinch bolts (with the knowledge that the crank arm is scrap anyways, but don’t strip the threads!!!), and finish the ride. Definitely avoid massive sprints. When you get home you’re pretty much replacing the entire drivetrain anyways, so even if you damage the right crank arm by overtightening it’s not really a big deal. Heck, it might actually be cheaper to buy a full drivetrain kit rather than buying all the parts (minus the right crank arm) piecewise.

300km isn’t crazy far—that’s what, another 2-3 days at most? I think you’ll be okay. It’s not like you’ve got another 3000km to go. When you get home, you can figure out what went wrong. Who last installed the crankset? Regardless, this is also a good reminder to check the torque on all fasteners before heading out on a long journey.