adding commercially available mycorrhiza, unfortunately I can't recommend it. There have been many studies that show that those mixes don't usually make a difference. This could be because the selection of species is not tailored to the plant or ecosystem, or simply because the mix isn't fresh. In my own preliminary greenhouse study where I tested 3 different commercially available mycorrhiza mixes, none of them were better than the sterile controls. There's also a concern about introducing fungal species that could be invasive or problematic, since we just don't know that much about AM (arbuscular mycorrhizal) fungal community composition. In my greenhouse study where I compared seedlings inoculated with retired cranberry bog soils, soils from newly restored Coonamessett, and from intact cedar swamps, overall the seedlings grew totally fine with the Coonamessett and the cran bog soils, overall even slightly better. Keep in mind my study was limited to 6 months of growth and of course limited in other ways. Overall my assessment is that there are plenty of AM fungal propagules in the soil at restoration sites (and probably also in the potting soil you use to grow the seedlings).

Here's an example of a review article on this topic: https://doi-org.esf.idm.oclc.org/10.1016/j.apsoil.2021.104225