The reliable and the live sync methods are different things, so the honest answer names both and points you at the stable one for your stated want:

The reliable method, export and import, stable because it depends on nothing staying connected: at contacts.google.com choose Export, select Google CSV or vCard and download. In Thunderbird open the Address Book and import the file through the address book menu or Tools, Import, choosing the file. This works today, next year and after any Google change because it is a one time file transfer with no live connection to break, which is exactly the reliability you asked for. Choose vCard from Google where offered, since Thunderbird maps it with the least field fuss or CSV when you want to inspect and trim columns first.

The field mapping moment: a CSV import walks a mapping screen pairing Google's columns to Thunderbird's fields, a minute of attention deciding whether phones and emails land in the right slots, while vCard usually self maps. Worth doing carefully once for a clean address book.

The live sync alternative, named honestly since you preemptively distrusted it: the TbSync add-on with a Google provider keeps Thunderbird and Google contacts synchronized continuously, edits flowing both ways, genuinely useful when you want one address book reflected in both places. Its cost is exactly your concern, it rides add-on maintenance against Google's API changes, occasionally needing an update after Google adjusts something. For a one time move it is overkill, and for your reliability priority the export and import wins, but if you later want the two kept in sync rather than copied once, TbSync is the tool that does it, accepting the maintenance tradeoff.

For the stated goal, moving contacts in reliably, the export and import is the answer, done in five minutes with nothing left to break afterward.