15GB is tiny, db selection doesn't really matter at that small of a scale

Depends.

15GB on an actual server? Peanuts.

15GB MariaDB/Mysql database on a Raspberry Pi SD card? Recipe for disaster.

Why is that?

RDBMS isn't the best place to store time series data. There are better and more efficient options for that.

I'm a noob when it comes to DBs, but stumbled upon using InfluxDB with Home Assistant - would you say that's a solid choice, or are there better alternatives out there?

InfluxDB is pretty much the most popular and widely used time series database.

The difference between just storing every value ever and a time series DB is that the latter one can reduce the data frequency when it gets older.

Like if you're measuring your fridge temperature, you might store it every minute. But do you care about 1 minute accuracy when the data is two years old? Would 5 or 15 minutes be enough?

This is what time series DBs do automatically.

They're also optimised for data that's formatted as <time stamp> - <values>, making inserts and queries fast for data like that.

Timeflux or some such are better alternative to store timeseries data.