The code sample you provided is not nested.
Our experience with the official BBCODE plugin was that it does not “play well” with nested BBCODE. However, if you do not have nested BBCODE, this is “more-than-likey” not an issue for your migration.
Our site migration had decades of nested BBCODE (hundreds of thousands) and it was a huge problem (even with the official BBCODE plugin, which we use today) so, we just stripped all the color (and some other code tags we did not need away). Not one user has complained text color from a post 5 years old is missing and nor will they.
Getting migrated BBCODE to work properly is NOT so simple like “use the official BBCODE plugin”. If a big migration of years of various combinations of nested BBCODE was that simple, our full migration time would have been weeks versus months. It’s something to experience first-hand and is not “a theory”, it is just how it is. The official BBCODE plugin is nice, and we use it, but only after stripping out much of the nested BBCODE.
We also made the decision to strip out all color BBCODE tags (completely) because when a color is hard coded into the text, it might look OK with one theme (a light theme for example), but the same color will not look good, and will often be a distraction on another theme (for example a dark theme), so we made the decision to strip all the color BBCODE tags and we are very happy we did.
Of course, each site has to make their own tradeoff choices during migration. I can tell we had some “very smart REGEX guys” writing for months to get our migration “perfect” and most of the time spent was because of BBCODE. The official plugins and gems for this were a good “first approximation” but left a lot to be desired in the final migration, so we wrote a lot of Ruby gsub() REGEX to clean it up.
Best of luck and hope this practical view “from the battlefield of migration” was helpful. There is no doubt that migrating from one legacy forum to another can be very “tricky” if your goal is a “Great Looking” migration versus a kind of “sloppy migration”.