It sounds like it might be ideal for you. I must admit it was quite a steep learning curve for me, but for you, I suspect less so! I’ve never used TeslaFi, but TeslaMate does also include an option to import your TeslaFi data to TeslaMate if you want to.Nice. I didn't renew either of my Teslafi or Teslastics subscriptions because I felt they just lacked features. One big one for me was total charge cost, for example, and general overview/dashboard totals. Not to mention the UI for Teslafi is god awful.
I'm fairly ok with reading databases and PHP, Ajax and such so might be an interesting project. I don't mind the data being logged if it's free if I only get around to the above later on...
Certainly an excuse to get a Pi as well!
Adrian Kumpf, the developer, releases an update every few weeks, adding new features and bug fixing. If anyone does try it and then go on to keep using it, I’d encourage you to donate a few Euros to Adrian to help support the project.