If I was designing my own BMS......
I would go with a controller:module architecture where the brains of the operation are on one PCB with multiple communication ports for various functions and features. While most of the control and monitoring can be done over common signaling schemes from RS232 to CAN and many inbetween......critical ALARM level signals should be conveyed over a dedicated line what will trigger a disconnect when any device asserts a failure. This allows the system to maintain a safety net even if/when the primary control system has failed.
Critical functions should be as analog as possible or at least programmatically isolated from the main application code. This would include over current protection and over/under cell voltage among others. You should expect the basic safety mechanisms to be functional even if the uC blows up and smokes out.
As for load switching.....MOSFETs are the only answer IMHO. Relays that a load break rated are big and expensive, however if the primary switching mechanism is MOSFET based, a mechanical relay that is NOT load break rated can be used for positive isolation with higher voltage systems. Those relays are much smaller and less expensive. High side switching with MOSFETs is trivial and it puts the designer in a position to use VERY high performance devices with sub mOhm RdsOn. If number of those in parallel, very high current handling is possible with very modest thermal rise.
Linear Tech (Analog Devices) and TI makes some exellent BMS control devices. If you are a glutton for punishment, you could go more discrete but I cannot imagine there are many good arguments for not using extremely well vetted controllers. Both companies also have excellent options for passive and active balancing. These devices will continue to work perfectly even if the controlling MCU is dead.
For current monitoring.....please integrate the shunt with the ADC circuit. Victron uses this architecture for their Smartshunt and it delivers enough performance that you can actually count on its energy accumulator data. TI has recently developed the INA228 which can be used as the basis for a very accurate energy monitor that is also rather simple in practical design - as long as it is very close to the shunt. It is a 20bit ADC and if you want to make use of that resolution, a tight design is needed.
The main controller should have com ports for cell monitoring, an optional balancer, charger input port controller, output switch port controller, shunt, temperature, heating pad controller, optional remote display, optional WiFi/BTLE connection, optional hardwire Ethernet. That way, you can configure something very basic all the way to ridiculous.
