The game is already Pay2Win for hiding Engineering behind the Horizon Expansion paywall... So, I don't see your validity.
I think the main difference between what you are suggesting and buying horizons is that horizons is a one-time purchase.
It's very hard to level the accusation of P2W against paid expansions because you only need to make a single payment (of a specified amount) to permanently unlock all of the relevant content with the associated increases to the player's power level. Additionally, the paid expansions come with other content that does not increase the player's power level, but rather gives the player something to do with their increased power level and provide a sense of progression (ED doesn't do the sense of progression that well imo).
In contrast, most of what is commonly defined as P2W is not limited to a single transaction. A player could make a single transaction of a few dollars for a minor advantage or they could make as many as a few hundred transactions totaling several thousand dollars for a massive advantage. Both players are ostensibly buying the same content, but the player that spent more money buying the content will have an advantage over the player who spent less money.
To summarize, there is nothing wrong with offering players the option of buying game content provided that:
- A significant portion (at least 50% - 75%) of the new content is not there for the express purpose of making the player stronger
- The content is permanently available after only a single purchase of a set price
Note that the additional content will still need to be reasonably priced for what is being offered (none of this 'dividing a full game into several parts and selling them as DLC' nonsense).