The best way in my opinion is to use a USB-C connected SSD.
It will NOT be nearly as fast as the internal, mind you.
For that you'd need an actual Thunderbolt 3 SSD.
But the latter would:
a) Cost A LOT more
b) Not actually be as fast as the internal (but be close)
My Crucial X8 drives give me a score of 900MBps on BlackMagic Disk Speed Test.
The internal drive on my 16" MBP is about three times that.
A Thunderbol3 OWC Envoy Pro claims 2500MBps, so it is nominally much faster than USB-C and close to my internal.
That said:
The X8 does not slow me down at all. It cost me something like 150 euros for 1TB. It costs 130 euros now.
A Thunderbolt 3 OWC Envoy Pro is 250 euros.
Would I be able to notice the difference in speed?
For me, the answer is no, I wouldn't. I would rather get another 1TB instead.
For you, it is your choice. Only you know if that extra speed is worth the extra cost.
We are at a point where the drives are no longer a bottleneck in most cases.