Here's an approach / guardrails, that have worked on the past few projects:
1. Break out only the things that need to get scaled
2. Think evolutionary not revolutionary
3. Define the problem space before coming up with any solutions and get agreement / alignment on the problem ( I can't tell you how many times people will bring solutions to problems that don't exist or because of a shiny ball )
4. consistent improvement to shorten testing and release time i.e. monthly to weekly to semi-weekly etc.. - this will allow the remaining services in the monolith (or bundle) more aligned with SOA