I daresay you need a larger SD card. I replaced the stock one my phone had. 16GB was not nearly enough, my music collection alone is 4x that size. I tossed in a 512 and it's been pretty good so far.
The absolute best external setup I've used (in large parts due to VNs like this) is a 1-2TB NVMe drive in a Sabrent external enclosure. It came with USB A and C cabling, no need for extra power, blazing fast, and fits so easily in my pockets I can forget about it.
(I feel like an advertisement!)
The only real problem I've had is that I can't connect it to my phone. I need a multi-port hub that's purely USB-C to facilitate this hobby.