The biggest performance boost you will see is from changing to a SSD from a hard disk.

Any SATA based 2.5" SSD will work fine. From the links shared by shinichi88 the image of the hard disk seems to be the 3.5" desktop HDD type - which I find suprising since AIOs usually use laptop parts. It could just be the wrong image is used, but even if its a 3.5" HDD, then you can still mount a 2.5" SSD in there with an adapter bracket (might not even need this to be honest).

But as others have said, the CPUs in there are still underpowered so don't expect too much. But basically what's happening right now is that due to such small amount of RAM available, the OS is using the HDD as memory (you can google what 'swap' memory is to understand more). As SSDs are magnitudes faster than conventional HDDs, having an SSD would improve this situation. You can also upgrade the RAM to reduce the amount of times swap space needs to be used/accessed, but some amount of it will always happen.

It will improve tasks that are not so CPU dependent but require memory/storage eg browsing with many tabs open, using office apps with multiple documents/spreadsheets open etc etc.

One good thing about the SSD is that say if you unhappy with the performance uplift, you can just remove the SSDs and use them in some other system - say if you buy new PCs to replace these ones you can use the SSDs in those.

But with the RAM, you likely won't be able to reuse.