I think it would depend on how much you know! One thing I would do is look up the complexity rating and the mechanics listing of a few games that you’re familiar with on BoardGameGeek (BGG) and then use that as a way to get a general idea of what might be a little bit more difficult to play.
For example, lets say you already know how to play Red Cathedral. As of this writing, on BGG it has a complexity rating of 2.81.
The expansion, Red Cathedral: Contractors also has a complexity rating of 2.81 but it adds a Worker Placement mechanic. If you already know Red Cathedral, I wouldn’t expect this to be too hard to learn.
Similarly, The Castles of Burgundy has a complexity rating of 2.98 and has quite a few different mechanics from Red Cathedral. Even though The Castles of Burgundy is quite a different game, it has a similar complexity rating so I would expect this to be a good choice for learning a bunch of new mechanics.
Generally, board games share a ton of mechanics in common. After you become familiar with a few of them, you can sometimes use them as shortcuts when learning other games. For example, I often take note that the deck shuffling mechanic in many games often match the deck shuffling rule from Dominion or that the worker placement mechanic in board games have a few variations but they all follow generally the same rules.
In my (probably controversial) opinion, once you become familiar with many different mechanics, your board game literacy improves and it becomes much easier to play games with higher complexity ratings on BGG. This is because games with higher complexity ratings on BGG tend to combine many mechanics rather than just using a few of them like in lower complexity games. Notably, complexity on BGG isn’t equivalent to how high the skill ceiling is for the game.
For example, Go only has a complexity rating of 3.93 but I would say that it’s a game that has a significantly higher skill ceiling and steeper learning curve. Despite its simple rules, I think you have to practice quite a bit before you feel like you know how to play the game. Lacerda games on the other hand have a lot of mechanics and rules but don’t feel as difficult skill-wise, despite having higher complexity ratings in the 4.0~4.5 range.