If writing an extension is not an attractive / feasible option for you, I would also suggest looking at some third party connector tools that give you greater flexibility in the mapping process (choosing standard or custom tables/entities, choosing the direction of flow, working with custom fields, more rules and conditions, etc.)
So you know, I am not affiliated with any of these solution providers, and am just sharing them so you can review and decide for yourself:
1. eOne Solutions SmartConnect
2. Scribe (they have two different solutions)
3. Kingswaysoft (I'm not 100% sure if their solution is ready for Business Central yet; it does work with NAV, the on-prem edition of Business Central
Each of these tools has some pre-built connectors for Dynamics 365 for Sales (the standalone CRM solution), and the ability for you to customize it.
I hope this helps. If my response has answered your question, please verify by clicking Yes next to "Did this answer your question?"
