Hypothetically, creating a private chain on Geth, where I would make sure in the Genesis that mining is easy enough to mine with phone (not thinking as a full time thing, but let's say, you could potentially mine your own transaction within reasonable time). What kind of implications in terms of security would this have if the network is scaled up to thousands of nodes? (easier 51% attack I assume). Alternatively, would making the chain PoS better, assuming coin is worth something and enough nodes on network?
One way to see it is just as a scaled-down version of the current EVM. As you said, 51% attacks would easily become a problem if mining is easy and there are not so many nodes (thousands). Maybe someone runs it in their GPU server farm instead of mobile phone as intended?
I imagine forks would be a lot more common and longer as block are easy to mine. You'd probably need to require a lot more confirmations before accepting any "result" from the network (at least results which involve assets).
If the network can be thought as a scaled-down version of the current EVM, the PoS might make equally much sense. Probably not 100% PoS, but some percentage.
Someone else has probably different angles to this.