I’ve built quite a few portable solutions in the past. The beauty of the peplink tech is that it is very modular. You can have a core device and add other peplinsk to it to increase the WAN count or to access wireless networks see Adams crazy 100 WANS case study here 100 WANs of Ultimate Performance - Peplink
Personally I would recommend the BR2 Pro or HD2 MBX 5G as the core device for portable use, then deploy transit duos or BR1 Pro 5Gs as the smaller bolt on WAN capacity options.
With a bit of thought and planning, every single case could be self contained then when deployed and tagged correctly in incontrol you could assign one as the primary gateway and the rest as its WANs quite easily.
I would typically breakout https://app.diagrams.net/ at this stage and start throwing together block diagrams for the different use cases, then after I would use my library of peplink devices and drop the most appropriate models in.
Then I would look at the use cases, find overlap and make a decision about which models could be lower or higher spec and become multi-purpose / multi-use case.
Happy to do it as a project with you if you like. Drop me a PM.