The best I can suggest is GOG which allows for downloading DRM free(ish) installers. They come in 4GB chunks:
https://imgur.com/lLxVlqUPerhaps, if you have a friend with uncapped internet they could download it for you (I know I used to do it in the past). Or you could get couple friends to download different parts in a month and then combine them.
As others mentioned the game does get some chunky patches every couple months, but with the manual installer you could not update immediately and download the update over time (or through friends) (not sure how those are handled through manual installer - current offering displays small hotfixes only, and installer itself is patched to the last major patch. It is not impossible that you would have to download whole installer again, and reinstal the game for the next major patch.)
If you never used GOG before - they do have a client called GOG Galaxy through which you can manage and download your library, but if you login to their store through a browser, you can download installers (like in the picture) and instal the games offline. Plus, you don't need a client and can keep the installer for the future - downside you need to download a chunky installer and than instal the game which will almost double the required disk space (you can of course erase the installer afterwards if you are not interested in keeping it).
If you are interested in Larian's previous games, you can get them there as well and download the same way. They shouldn't get patches these days (not any major ones at least), so once you download them, they shouldn't require further data.
Unfortunately, with higher fidelity games became chunky and highspeed unlimited interned became almost a requirement for modern gaming.
