I can't believe they would charge that amount for data. I was was pulling solar data in the past as well - pretty straightforward. To be fair it probably took you more time to set up but now that the work is done and maintenance is probably pretty low it should pay off after a couple of years.
$199 was a bit steep. If it was a one off, may be I just would have paid it, but since it's an annual fee, it's totally worth it.
Honestly, most of the solar / power tracking, graphing etc for home-assistant now works out the box[0][1]. I literally installed a plugin, and changed a few check boxes in the config. Not saying it's trivial for anybody, but for me it was. The real time data has proven invaluable in reducing power bills by knowing when we're importing or exporting power (1:3 price ratio here), and used that to load shift the expensive power users.
Since then I've added some temperature & humidity tracking to work out which rooms are letting the heat/cold in and out, done some basic automation to know when the washing machines and dryer out in the garage have finished, and a sensor to tell me when the garage door is opened or closed. I used a combination of BLE sensors and ZWave.
What other projects did you look into?