Hi gumball,
Latex layers and mattresses come in a very wide range of firmness levels from ultra plush to ultra firm and everything in between and there are many side sleepers (including me) who prefer a latex mattress over any other type of mattress or material. It’s certainly not accurate that latex is “too firm” for side sleepers (most “broad brush” comments like this aren’t usually accurate) but the most reliable way to know which types of materials or mattresses you prefer or whether a specific mattress (latex or otherwise) is a good “match” for you in terms of PPP is based on your own personal testing or experience.
You can see my comments about the wire grid platform foundations in post #10 here.
You can see my comments about some of the additional risks of a solid surface support system in post #10 here. Plywood can also warp or sag over time if it’s used on a sagging or flexible surface (see post #11 here) especially if it doesn’t have good center support so I would keep an eye on it if you do decide to go in this direction.
Phoenix