The thing about the expanding foam is it may force you to get a new post anyway.
When that stuff expands, it can push cracks apart and if you have enough hollow in that center, it may be enough to provide decent compression power to the foam.
On the positive side, the foam will keep water from going in there, which is causing/exacerbating the damage.
Will foam provide support? Marginally. Bolts won’t get much added strength, but the entire post may flex a tiny bit less, assuming it doesn’t split during the curing period.
Silicone may work better, but I’m betting the quantity needed would surpass the cost of having someone replace it all.
You won’t be out much if it doesn’t work, but if it does end up splitting the wood, you may want to be prepared to replace it.