You can have multiple fields in one measurement; they will all have the same timestamp. The Sample Data in the InfluxDB Key Concepts documentation uses points with two fields and two tags.
Additionally, InfluxDB does smart things to compress the data it stores. I’m not sure if that was taken into account in your calculations.
Generally, time series data is less valuable the older it gets. Many users will “downsample” their data; they will keep the last two weeks of data at 10 second resolution, for example, but for data older than that they might only keep data at a resolution of 1 minute.
Do you need 10s resolution for all of your data, forever? In that case, you should probably buy a bigger SD card
Otherwise, I recommend checking out our documentation on Downsampling and Data Retention.