Nicely done. The most activity I've seen for a while mate.
Sometimes not thinking about it is the best solution ;-) Avoidance avoids worry ;-) Whatever you read, there's black and white and usually no really reliable and objective data to base an opinion on.
My tyres were used originally for Potatoes (they were all the rage for Potato gardens at the time) then hung about the yard for some years. We put them out with the kerbside cleanup and they were left, seems we have to pay to get rid of them. Now they are used to hold up several pots of Bromeliads, stops the pots falling over. I've one around a young Coconut just as a garden edge.
Most of we older people never thought much about the toxicity of these things we took for granted (tyres etc). For better or worse, if we're polluted with whatever-it-is it's more or less too bad for us.
For younger people, looking into these potential toxins is quite a good idea. Especially if avoiding them is practical. The less toxins we take in the better imho - everything we eat has to be metabolised via our Liver.
There has to be many reasons why there are epidemics in the western world of diabetes, obesity, cancers, heart disease and allergies to name a few. Perhaps avoiding as many toxins as possible is one way of staying that little bit healthier.
An alternative to timber is to buy or make concrete sleepers. There are a few people who produce these commercially and you can cast your own too.
We picked up a couple of used steel moulds on ebay and any sheetmetal shop should be able to make these.
You could also probably make some timber & plywood reusable forms if you want to cast your own (the advantage of making your own is being able to make them to custom lengths). You need to make them 75-100mm thick and put (at least) a couple of Y12 (N12) reinforcing bars in the middle (hang the bars from gal tie wire). Tap the form to compact the concrete and get rid of air bubbles.
The downside is they are heavy and you will need someone to help you assemble them.
There are special posts with slots you can get for building raised beds, retaining walls etc. But you can just drill through them with a rotary hammer drill and bolt them together (I would suggest you use stainless fasteners such as M12 threaded bar).
Zinc is an essential trace element to plants (and us!). Given the thin coatings (and therefore the small amount of zinc) used in protective steel coatings I doubt there is any risk to gardeners via their produce.