Well, you own the game already. so just pick which ones you may thing you would like according to your playstyle so far in the game.

Each major expansion has a few things that are useful.

But you only need the base game is all.

These are the major expansion and what they do, with videos trailers. There are 14 of them to choose from.

https://skylines.paradoxwikis.com/Downloadable_content#Expansions

You can look through them to find minor Content Creator Packs (assets packs) and map packs to add to the game

I would say the Mayors’ Favourites Bundle is probably a good start. The expansions offered with that bundle seem, overall, to be among the most popular by a broad segment of players, particularly After Dark, Industries, Mass Transit and Parklife. The bundle is light on items such as radio stations and content creator packs, allowing you to choose your own.

The one element that would be missing (for me) from that bundle are trams/streetcars available through the Snowfall DLC which is not included. But otherwise it is a quite good set of early DLCs that added generally popular elements to the base game.