You can change the lens mount to PL. Swap in something like a PL KipperTie Revolva and you essentially get very close to internal ND.

There are chassis that let you externalize additional GPU's over Thunderbolt 3 and/or USB-C. Consider looking at resources like https://egpu.io/. There are a lot of downsides to the eGPU reviewed here... it's built around a mid-level GPU and is not upgradable. Also consider which software and which processes within are optimized around additional GPU's; Resolve in general loves extra GPU's where as Premiere and AE could care less for the most part.

B&H's specs show a 4k bit rate of 1 Gbps at 12 bit up to 29.97P and 10 bit at 59.94P. 150 Mbps is in reference to UHD 8 bit MP4.
https://www.bhphotovideo.com/c/product/1340800-REG/canon_2215c002_eos_c2...

With the added shims option, you should be able to switch between them yourself.