--that B12 level is optimal; many have reported having neural symptoms from B12 deficiency even into levels of around 700.

In most nations other than the US, the lower level of B12 adequacy is often 500-550; the standard US lab range of 200-900 or so is likely an artifact from days gone by before there was much more research into how common B12 malabsorption was, especially among older people.

It might not hurt to take some B12--1000mcg/day, in methylcobalamin rather cyanocobalamin form, just in case you are one of those people with methylation issues (that way the B12 has to jump through fewer chemical transformation hoops to get to the tissues).

Obviously, you should have your ferritin (not just serum iron) and Vitamin D3 levels checked regularly as well, given what happened.