Hi Steve,
1- There is another discussion on the community forum similar to your question and has a few suggestions there. It can be found on->
How to find similarity between two graphs in Neo4j?.
This might provide you some options to try out and see what best works.
2- GDS (Graph data science) library of Neo4j also provides out of box support for some similarity algorithms that could used, which one best fits is subjected to the exact use case you want to achieve.
3- Apoc also provides graph comparison algorithms like fingerprinting, around which a solution can be created.
What would exactly fit for your use case is subjective to the problem, the solution looking for, the neo4j db performance, etc. I would recommend you to go through all these option and take the best 2-3 options and try them over a POC to see what best solves your problem and is also efficient with your current neo4j configurations.
Regards,
D