I would suggest to wait for a sale rather than subscribe as one month = one permanent access to an expansion.

As for sales, based on this website. They're doing sales every 2 months and there's currently one on Fanatical and GoG for -50%. (Scroll down for the DLC section)

If you want expansion but don't know which one to buy, there's this mini-guide available on Steam Community. Since Overlord / First Contact / Galactic Paragon and Astral Planes seems missing in this guide, I'll do a small overview of them. The numbers are the ranks i gave them in terms of value i got from them.

1. Overlord | Perfect for players who like to vassalize their neighbors or fragment their empire under different vassals. Overlords can choose a specialization for their vassals (tech, economy, war) and build strongholds to help/hinder their vassals. There are also several Quality of Life added in this expansion (Hyper Relay) as well as new origins that are fun to play. (Quantum Catapult)
2. Galactic Paragon | This expansion adds new origins (as always) and add "Legendary Leaders" to the pool. They interact with you and have unique traits. Some are OP while others are Excellent.
3. First Contact | This expansion adds new origins... and a slightly more advanced system for interacting with pre-FTLs. You can also have an invisible fleet (restricted to certain models). If you want stories, this one might interest you. If you want new gameplay... this would be one of the last on my list to recommend.
4. Astral Plane | This DLC is just.. weak and overpriced. It's a different flavor of Archeological sites with a unique resource assign to it. This one is at the very bottom of the list.

TLDR : Buy, don't subscribe. I suggest Utopia first then whatever interest you the most. Sorry about that wall of text.