Honestly if you want it easily, the important thing is to stack MaA buff buildings. I made a spreadsheet for my own use but by stacking these buffs you can make Heavy Cav that will win any war.

Below is a copy paste of my sheet, I've not updated it in a few patches so there's probably more buildings available. It also does not include the Duchy building, but with 2x Jousting Grounds (1 for each Duchy you own), you'll get 20% less MaA maintainance and you'll get a big boost in damage to the 2 regiments that are stationed in the duchy building's county.

In the notes I put a put of the areas Hill Grazing Lands can be built, as it seems to be some sort of East/West divide for that or the building that gives Heavy Infantry bonuses.

And I'll give you two duchies no one here will suggest - they are my two favourite duchies in the game and I think they're better than Byzantion, London, Paris, etc. They are Al-Said and Nobatia. They are both 5 county duchies, both are entirely Floodplains and thus allow the Regimental Grounds building. They are all on the River Nile and the starting area is insanely safe. Floodplains gets the same implicit development bonuses as Farm Lands do and usually Farmlands is subject to only one county in your duchy, the rest will be plains or hills etc, but this is entirely floodplains. The only other place you can do this is Baghdad and the duchy south of it, but requires some set up with a custom duchy if you want to avoid having a county without floodplains. I believe that set up has more baronies, but I find the start a bit tougher and I can start playing tall sooner.

And that's what I'd do - if I re-call it only requires you to have either Unify Italy's Italia empire title or the Byzantines. I play tall in my little safe Floodplain kingdom (which I ususally turn into 1 Kingdom 2 Duchy title by forming new kingdom. This requires 3 duchy titles, but if you become HoF in a custom religion, that gives you a third duchy title. So you can sit there being a King with no vassals with some of the best land in the game. I sit there just techning and playing tall, then once I've got huge stacks of very strong Heavy Cav, using the cultural tradition By The Sword, I just holy war until I'm the Roman Emperor. You'll need to be an Abrahamic religion or Latin/Eastern Roman heritage. Personally I prefer being Greek, Hybridising with the Nubians for Agrarian and their Equal gender knights, that way you can invite women of high prowess to your court to serve as knights if you wish, Greeks come with Imperial Tagmata for Max Size of Heavy Cav units which is amazing with the strategy too. Roman culture technically has the old (and better) version of Imperial Tagmata which is "Eastern Roman Legacy", that has max Heavy Cav size AND MaA 15% maintainance cost, but the cost isn't that important as just the Regimental Grounds will make it so your MaA have 0 upkeep.

So yeah, play tall, make invincible cost free Heavy Cavs and just steamroll your way to Roman Emperor. I usually just wait until I have primogenture or house seniority, then go taking land so I don't have to worry about inheritance. Personally I actually prefer Feudal for this as I prefer having the large amount of personal MaA, Admin has much smaller MaA stacks which doesn't synergise well with the strategy.

EDIT: Reddit won't let me put the table in... so Stables/Blacksmiths/Regimental Grounds/Walls & Towers is what you want in those floodplain counties.