A couple minor updates after checking things out some more.
1. The bolts appear to be 5/8”.
2. I can’t go with the green plan from earlier, meaning using the same hole on the posts for two different 45° braces. It works in the current config because one is on the front of the post and one on the back. But the back (house side) of the post doesn’t line up with the horizontal board the brace would need to attach to. No big deal, just have to drill two extra holes and stagger two of the braces a bit.
3. The posts are not in concrete.
...which leads to the one major issue. I dug around the one on the corner since it was easily accessible to see if I hit concrete. And find this:
View attachment 253646
When I dug down to see if I get to concrete, or the bottom of the post, and it looks like it went 4-5” into the ground...as there were small roots running under it below that depth.
View attachment 253647
So I guess replacing a post will be step 1. I dug a little around the other three and didn’t see the same rot. I’ll assume water runoff on that corner was a contributing factor.
At least I now have a plan in my head to proceed.