Crusader Kings III

Good guide. I tried your idea of starting as Duke Antso of Gascogne in 867 and wanted to share my findings. Your suggestion to do it in the aftermath of another campaign when you already have a big empire before pushing into Iberia is certainly easier, but Antso is a good choice if you want to get involved in the struggle quickly and grow within Iberia. I had already done the Iberian hostilities achievement during my Jimena siblings heritage war campaign, so my plan was to go for the compromise ending this time and pick up the "Basque in my glory" achievement on the way. A campaign in which I wouldn't conquer a huge empire for a change, but just be the protector of a stubborn little mountain folk who want to do their own thing and just want everybody to leave them alone ;-).

Antso turned out to be a good choice for that. He has military education and quite good values. The Basque culture in 867 starts underdeveloped (still stuck in tribal era, despite already having feudal government), but it has a special tradition that allows you to pass the high partition law. This allows you to stay a vassal duke to West Francia / Aquitaine for protection and conquer the territory of several duchies until you are strong enough to defend yourself against your strongest neighbors on your own without having the risk of your realm falling apart on sucession (as long as you don't create the additional duchy titles).

Due to his starting territory being underdeveloped, Antso is weaker and poorer than you would expect from the size of his realm, but he is still way stronger than Navarra, so just fabricate a claim on Navarra, conquer it and move your capital there. Now you are involved in the struggle and can use the struggle clash casus belli to expand further into the rest of Navarra, Barcelona and/or Asturia without needing to fabricate claims. You'll need allies, though, because all of your neighbors except Navarra have equal or higher military strength than Antso and there's not much you can do about that with your low income and low technology level.

Antso has one major drawback as a starting character, though: He starts with an almost 50 year old wife and only two sons. The Basques have the sacred marriages tradition, which means you can't ever divorce, murder schemes against your spouse are extra difficult and adulterer traits are despised, so Antso can't easily get more children for marriage alliances. The sons are at a good marriage age (one a young adult, one 15). I married the player heir to a landless genius to get a better heir in the long term and used the second son for a marriage alliance with Asturias.
Then I gave both of them a county, since Antso was already above his domain limit and the heirs develop better when they have something to rule, so landing them is usually a good idea. Yet in this case, it was a huge mistake, because landing them meant I couldn't marry off their children for more alliances. I should have left them unlanded for a few years to arrange tactical betrothals. Due to this mistake, I was stuck with only one allie, which wasn't enough to expand further, so I had a very slow start. I basically bid my time making money by blackmailing neighbor courts until Antso died and the heir took over.

The heir and his wife had fortunately bred like rabbits, so now I could get some more allies and really get going about conquering the whole northeast of Iberia. Once I was strong enough to stand on my own and couldn't have more counties as a duke without penalties, I made a dissolution faction against Aquitaine (so I would have no northern neighbor strong enough to threaten me), won it with the old kidnapping trick and founded the Kingdom of Aragon (I could also have initiated a coup to rule Aquitaine instead, but I wanted to concentrate on Iberia).

I then took the decision to embrace the Basque pagan faith for the "Baque in my glory" achievement, which was another mistake, because the Basque pagan faith is curiously not regarded as involved in the struggle, so it turned me into a mere interloper. The Holidaying in Iberia achievement wasn't lost, though, I just had to undo that decision by having my heir convert to an involved faith again before ending the struggle, so it was just a minor delay. I had decided to leave the Basque pagan faith as it was, so I don't know if it would have counted as involved if I had instead initially decided to reform it (which might have made it a faith that was "created within the struggle").

In the end, I got the achievement by ending the struggle through the compromise ending. I'll also write a guide how to get the compromise ending under the "Iberian compromise" achievement.