I’m honestly not sure if the developer edition is required to make custom plugins, I got the Clarity Kit upgrade for the battery upgrade, which apparently also includes the developer edition. Probably worth reaching out to them for clarification.
As for my experience, their web UI for making private plugins basically lets you provide an API endpoint for data and an interface to paste in templates (using Liquid templating). So all of my logic is completely outside of the TRMNL system in a custom API I mostly vibe coded and am hosting on a cheap server, which effectively gave me infinite freedom to build whatever I wanted and just have TRMNL handle the UI. So you could really use whatever language you prefer and just return JSON to the TRMNL. Since the logic was decoupled, I also threw together a web version using the same API.