In ~/.bitcoin/bitcoin.conf I set prune=5000.

That reduced the blockchain size to 5G.

du -sch /mnt/btrfs/bitcoind/*
...
5.0G   /mnt/btrfs/bitcoind/blocks
2.8G   /mnt/btrfs/bitcoind/chainstate
17G    /mnt/btrfs/bitcoind/indexes
....
25G    total

This is indeed a x10 improvement, before pruning, my bitcoind data directory was roughly 250G. Yet, it's still 25G total. I need to run on a small SD card where there is only 10G of free space.

Now /mnt/btrfs/bitcoind/indexes/txindex/*.ldb files take up the majority of space, 17G.

Is there a way to prune the txindex/*.ldb files?