Worth upgrading tonearm cable?

Get a Rega instead, if you sell the Linn and add the $150 you will probably be there.

Which Rega do you recommend? And would I need a new armboard for my SOTA?

Any, the 600 is better but more expensive. There is a new 250 with VTA adjustment for $350. The lack of VTA adjustment is the only problem with the Rega and it can be added as it is here. I cannot say about the arm board; it may fit in the same cutout. Check the length of each arm. If the SOTA uses a wood board a woodworking shop can make a new one and cut it for new arm if you need a new one.

The Rega is a slightly longer tonearm than the Linn, roughly 237-240 mm for the Rega vs. 229 mm for the Linn. If you switch tonearms you'll need a new arm board.