Well i would say wait until Horizons is released so before Xmas.
The thing is...i am sure there are exciting times ahead. You want to be in a good ship when the Thargoid Invasion Armada storms occupied space Now that could happen after Season 2 (horizons) and Season 3 (atmosphere landings) so in 3 years basically..but i dont think they will wait that long the fans will depart in droves..i reckon it will be part of the main game Elite and i dont think they will advertise it saying expect thargoid incursion on X date. I think buy it play it so you have a decent ship and some millions in the bank then shelf it until it gets the more interesting content.
There have been signs already ...they are coming. (i really hope they are big fans of Babylon 5 )
Where is your source that season 3 will be atmospheric landings? They have stated that Thargoids will not be announced so yeh, should be cool to see a few random encounters over however long they decide to take to introduce them Agreed with you on the fact that they will be part of the base game, it just wouldn't be right any other way.
As for waiting, it comes down to whether or not you decide that £30 is worth the cost of Elite as it stands (plus CQC and Ships updates) is good value for 3 or 4 months of gaming before you're in a position of needing to spend another £30 to get the Horizons expansion, bearing in mind that there is no requirement that you buy the expansion to continue playing, you just will not be able to take part in planetary landings.