Sodium ion batteries are already being mass produced in China. CATL actually just started producing their second generation sodium ion batteries. In the US, Peak energy is doing storage solutions based on sodium ion.

Anyway, you are comparing apples and oranges. While solid state sodium ion might become a thing at some point, it so far isn't. The lithium based solid state batteries currently being readied by several battery companies for mass production around 2028 or so tend to have up to 500-600 wh/kg densities. Sodium ion batteries are currently at or below 175 wh/kg typically. LFP is a bit better, and some high end NMC batteries might do 250ish wh/kg. That would be just the first generation solid state batteries. Densities might improve after that. The theoretical limit is a lot denser than that and there is a lot of money going into researching ways to do better than that.

Of course energy density is just one thing you might optimize for. Other properties you might look at are operating temperatures, amount of charge cycles the battery can handle before it degrades below 85% of its original capacity, the speed at which it can cycle, fire safety, cost, etc. Mostly sodium ion scores very well on all of this except density.

High energy density usually comes at a price. Both in dollars and in compromises with these other things. Think lower lifetime, more constrained temperature ranges, etc. Worth it if weight and volume are really constrained. Like in anything that flies.

Depends on the application - each battery technology has a trade-off between energy density, cost, lifetime, safety, scalability, etc. Sodium may have a place in grid storage, although iron-air is being deployed today and is even cheaper and safer (but poor energy density, which doesn't matter much for grid storage - https://formenergy.com/technology/battery-technology/)

Cheaper yes. Non-aqueous sodium ion batteries (which are most of them) are more dangerous than LiFePO4 though.