I feel like (for better or worse) caching/installs kinda have to be the way to go for faster performance. UHS-II is expensive, III basically doesn't exist on the market it seems (might as well be UFS), SD Express is a ways off (and will probably be expensive as well), and I don't see the game cards themselves becoming majorly faster either. Course with heavy usage like caching involves, you'd want big storage or expandable/replaceable memory to reduce the wear.
So I guess we're back to M.2 2230 seeming like the best option...size, power, and ruggedness concerns be damned.
eUFS is still very much an applicable option for replacing eMMC for internal storage at least, given how widespread its usage is in Android phones, which would definitely be the option if internal installs are needed.
For comparison, the original Switch used a Toshiba THGBMHG8C2LBAIL eMMC chip, which was rated to the JEDEC eMMC 5.1 standard. The random read rating of eMMC 5.1 is up to 11,000 IOPS (input/output operations per second). Samsung‘s eUFS v3.1 chips (which are now in mass production) are rated at up to 100,000 IOPS in the same random read category. That’s nearly a literal order of magnitude increase in read performance. Write performance is a similar deal. Even eUFS v2.0 (from back in 2016) is rated at 4 times the random read performance of the fastest eMMC.
eUFS seems like the choice for more performant internal storage, the only thing that remains is what external storage will be. As we discussed prior, Nintendo could leverage its position to push for UFS card production.
As for game cards, we know so little about the game card and its reader’s theoretical upper limits, or if there are any other viable ROM-based options Nintendo could opt for other than Macronix‘s SIMD XtraROM solution.
Internal storage is all sewn up, as far as I’m concerned, it will be some version of eUFS. How Nintendo resolves the rest, if they choose to resolve it at all, is the only question that remains.