I am far from expert, I took my MX5 to someone to take care of it, on the basis that before any underselling is done, it had to be prepped first. Wirebrushed off the surface rust, rust convert anything else, into the cavities etc, then it gets sealed after.
You're right to be cautious, I know a garage who see a lot of jap stuff and they have said since a lot of people DIY'd undersealing during the pandemic when they had lots of time and nothing to do, they've seen an uptick in rotten subframes with protection on top.
I am assuming that given yours is relatively new, it shouldn't be too bad underneath right now, but I would suggest you get a look under there and then either bone up on the process if you're doing it yourself, or try and find a good provider who talk you through what their process is. You can never be 100% sure, you have to trust your judgement.