Use Syvec and forget PITA with Simos 18 immo. CAN traffic more less same as Simos for integration.
Why would you spend $5000+ and need to retune a standalone ECU when you could spend a few hundred or less on a patch service and find someone with a bench tool or a tool that allows full write OBD (I hear CMD may allow this now but definitely bFlash or my open source tools), or spend $0 if you are decent at reversing to disable the immo lock entirely?
Even if you don't have a tool of your own or don't want to use open source, someone with a commercial bench tool shouldn't charge much for this and you could get away for a few hundred dollars all in.
Going to a standalone over this kind of thing might have made sense a few years ago, maybe, but with how well Simos18 is known these days, this seems like a huge waste of $.