So to return the favor.
So more aelves seems to be somewhat given in a current state to even be able to do 3 blobs for battlelines, but you probably would want to expand your leadership with that so you can have a hag per blob to make sure they are all brewed up and get a prayer.
But if you are going with more of both of these you might just as well go battleforce to reinforce all parts of your army imo.
it seems like a larger undertaking to switch to snake lady army completely and seems like a larger economic undertaking than any other option.
For allies it is a question of, do you want to make the punchy army more punchy or do you want to add in options to make up for the lack of range?
As I myself is currently (until I get my new ladies) is a stormcast player I see them having access to several options of ranged combat that could be of interest.
Ex a lord ordinator and two ballistas would be 340 points for what could be a quite devastating amount of firepower, or the vanguard raptors with hurricane crossbows that is doing a lot of attacks but also have a bubble of -1 to charge rolls that might help with board control.
Otherwise there is the what seems to be almost standard tank idoneth eels that I see a lot of lists running with that seems to be doing a good job
I do have one other idea: Mortahi