Battery relocation and kill switch

Good Evening All,

I have a 2014 Mini JCW and want to do a battery relocation and include a kill switch (With ignition cut etc). I have attached a diagram outlining the current setup and what my proposed setup would be.

I am looking for guidance/advise if my proposed set is correct, what guage of wire to use and do I need to include a circuit breaker around 200-250a on the battery positive cable before the kill switch?

Current setup: Two battery positive cables going to the starter motor and battery fuse box. System does have an ibs sensor which I will move to new battery location.

Proposed setup: A new cable running to a kill switch w/ignition cut, then onto the original cable to the starter motor and battery fuse box. On this cable a branch to the alternator exactly like the current setup.