Although pulling new cables would be the best solution, I certainly understand the hassle that can entail.
I also agree that a butt connection is preferable to a bus bar or post for the reasons DaveM raised.
And it looks like heavy duty but crimp connectors are available is big wire sizes.
https://crimpsupply.com/connectors/2...terminals.html
Just be sure to use a proper crimper with dies for each gauge. Not one of the hammer crimpers, or pliers, or a vise. And get the best shrink tubing you can find.