No, there isn't. The Definitive Edition was added as an update to the original game (so everyone would get it automatically), so there isn't a way to exclude the original release.

The install size should 'only' be about 59GB, if you can free up enough space for the install process.
The data for the Classic version can be removed, but with the next game update, Galaxy would want to re-download it. There are a couple possible ways to get around that, though.

Once you get the game installed, you could move the original release data folder to a USB flash drive, if available, or another hard drive, and then use a symbolic link to make the files and folders appear to be at the expected location for Galaxy, so it doesn't redownload them if you verify files or the game updates (in Galaxy, select the game, click More and select Settings, then uncheck automatic updates).
In the 'Divinity Original Sin 2' folder, 'Classic' and 'Data' can both be moved, as well as 'bin' since you wouldn't be using the launcher; most of the size is the 'Data' folder, though.
Creating the applicable folders before the install and setting a symbolic link to redirect to another drive would presumably get Galaxy to split up the folder as the game was being installed.

If you have a drive large enough for the game folder (even if it is too slow to be able to play from there), you could have Galaxy install the game there, and after that copy the '..\Divinity Original Sin 2\DefEd' folder to the main/faster drive.
That would avoid requiring any symlinks, but you would have to manually update the folder you are running the game from after each Galaxy update, and since the game can not connect to Galaxy if it is started from the executable, you wouldn't be able to use the online lobby or GOG friend invites that way (not sure about tracking achievements), or the cloud save function.