Hi @eccountable,
I believe Microsoft still recommend building a single unmanaged solution per development environment, so that would potentially mean multiple different development environments for each 'layer', and each one being promoted to the development environment corresponding to higher layers as managed.
If your solution segmentation is based on technical boundaries (ie. component types), this can become quite expensive and complex to implement, so the approach I have taken so far is to have all my unmanaged solutions built in a single development environment. The Microsoft recommendation could potentially be more practical if your segmentation was based on another dimension (eg. based on geographic region, or business process/functionality etc)
I have been trying to work out a clean separation/segregation approach for solutions based on component types in my head for a while. The following is the best I could come up with but haven't tried it yet, so not 100% sure it would work. Hopefully it gives you a bit of a starting point. The idea is to think about potential component inter-dependencies and plan the correct solution to place each component type in in order to deploy in a fixed order gracefully. So any additional component you introduce, would need to be planned into the correct existing solution or introduced into a new solution in the correct place. I haven't considered here things like SLAs, Templates and many other component types that you should consider.
Solution 1: Custom Connectors and PCF Controls
Solution 2: Data Model (Entity, Columns, Choices, Web Resources, Security Roles, Field Security Profiles)
Solution 3: Plugin Assemblies, SDK Message Processing Steps, Custom Workflow Assemblies, Classic Workflows, Custom Actions, Custom APIs
Solution 4: UI (Forms, Views, Charts, Dashboards, Business Rules, Business Process Flows, Reports)
Solution 5: Cloud Flows and Connection References
Solution 6: Model-Driven Applications and Sitemaps, Canvas Apps, Custom Pages
Hope this helps. Happy to discuss further.