I own TQ:AE on gog but none of the DLC

As you know, 1.45 was the last update before the release of the Ragnarok expansion
All updates since then are affected by in-game "advertisement" for DLC content
The game installation footprint has also increased considerably since then.

Unfortunately the Galaxy client does not let me rollback to such an early version of the game.

What are my options?
Should I write to gog support and ask them to add the 1.45 installer as a goodie for the base game?
Is such a thing even possible?

GOG support have provided (temporary) links to old versions when I raised a ticket previously, so there's no harm in trying (but of course, you will have to wait a month or so).

Having the "original" TQ:AE as a goodie would be a nice idea since you have a number of mods for it that won't work with the latest version.

Realistically though, your best bet is to retrieve a backup of the original installer - if you don't have backups, take this as a warning to start being religious over them, since any other game could be similarly affected. GOG isn't a cloud backup service. :(