You don't want to use an SD card "in place of" a REAL hard drive.
I'd suggest an external SSD. It could be:
- USB3 (will give read speeds of about 430MBps)
or
- USB3.1 gen2 (will give read speeds of 800MBps or better).
You can buy "ready-to-go", pre-assembled drives, or buy the components and put it together yourself. I usually "make my own".
But SOMETHING VERY IMPORTANT to bear in mind:
Once you start using an external drive as "primary external storage", you're also going to have to begin BACKING IT UP to a SECOND external, which will become your "primary external storage backup". The backup could be a platter-based drive (which is cheaper). But if you're going to keep any "files of value" on an external, those files aren't "safe" unless they're backed up.
I'd suggest either CarbonCopyCloner or SuperDuper for this job.
The backup will then be AN EXACT COPY of the source drive -- all-but impossible to tell apart (if you don't know how).