Basically, you want to get ride of all vassals as soon as you can with your first king to grab all the lands. In the 867 start date, its a bit harder because you have to murder them or make them fight until they die in combat, but still doable with most countries at the start. The 1066 start is easier because you have the ability to revoke title at the start.
The first step is securing a strong alliance with your closer bigger neighbor. Then revoke the title of one of your vassal that you have a claim on if he accepts move to the next claim, but if he revolt, he will most likely bring many more vassal in the war against tyranny. This is where your alliance pays off, call them on and clean up the country. When you enforce your victory every rebellious vassal will be imprisoned. For their crime against the crown you can freely remove their titles and own their lands without the tyranny debuff.
The warrior lifestyle is probably the easiest way to go since you will fight a lot and you will need that control buff of the authority focus after revoking vassals titles. Serve the crown and strict organization perks are pretty much mandatory. Once you start to have a decent real stewardship with the wealth focus and the tax man perk is very good. After that, anything is fair game.
To bypass the 90% debuff of holding all the lands yourself, you will need to rely on the religious estates which are not affected by it. Build farms and pastures in every one you own and make sure your bishop like you.
Simply build at least 1 level forts in all your castles and then when you have a strong economy you can build barracks in your castles since you own them all you will have a massive army in no time.
You don't want to have any vassal (barons of your towns) in you court as much as you can. You will have to use your courtiers to marry high level council members and knights. Since they don't own any lands they will not be affected by the insane debuf of owning all the lands.
Make sure you only have one heir using the disinherit before you die (you can still somewhat save scum in ironman since it save at the start of the month)
Play as Æthelred I and stress him out of the crown until he abdicate or die so you can start as Alfred And you don't have any vassals do deal with.
- Invite your sister Elvira back to court and marry her to Philippe of France.
- Marry good council members into your court and make sure to have a 20+ spymaster.
- Start a murder sheme against Alfonso and then Fernandez.
- Start moping up your vassals with the revoking title techniques as explain before with Philippe help.
- Once your done with the vassals declare war on Navarre for the crown.
- Mop up Galicia and Navarre vassals after you inherited.
- Unite the crown of Spain and onward for the Reconquista.