what general do you recommend

I'd say Karl.

"Auras" dominate the world of EW6 1806. If your units fight inside the aura of such generals like Murat, Karl or Mahmud they benefit greatly from this. Moreover these skills stack up, so, for example, a simple militia unit fighting within 3 auras of Massena, Karl and Barclay potentially has + 60 bonus in attack. The same is true about Cavalry and Artillery.

I used Alexander before. He is pretty good. I’d say the aura salvo and very rank is worth it

I personally recommend John, as I believe I saw in a post he has better output on Mt-Forest Tiles which aren't very rare). I also don't bunch my artillery together unless there is a naval invasion.

My first general was Karl, but he was too expensive for early game (sold him because of this), so I went with Massena. Massena has more output for me and didn't need much to upgrade.

So I would say Massena.

Hmm, now in a re-play of the game (YES, remember to cloud save or else one day you can lose all your data like mine) I find the ET has buffed jourdan and gates' starting ranks and clinton too i think. I'm not sure as the last time I played this game was last year as I rage quitted and moved on to 1914 after all my data was lost (I was in the middle of crimean war). 
But I'm actually considering upgrading jourdan to M General to equip 2 items? 
Also, now that you could automatically smack an item onto your free gens I think buying healing items first could work. 
But I think after buying murat and Mahmud you should go for Dabrowski and then get Karl (i think you only need 1 inf gen, 2 art and 4 cav to finish the game with nelson at the end) 
Remember to upgrade your tech tho! So tha you won't be bungling thorugh missions with 1 or 2 levels of lower tech (tho in my experience  OP gens can sometimes compensate this) 
But after Karl I think you should make murat OP as well as Karl upgrading mahmud's aura will also help. And then chase for Lan. 

S rank 1812 conquest. I got it by playing morroco. But make sure you almost have near-max tech for it, at the very least max cav/inf attack and almost max health and defence upgrades. By the time you attempt the 1812 conquest is when you have 1 art gen+jourdan, murat, karl and another inf/cav gen depending on your major (I had murat+davout). If you want a relatively fast conquest pick a coalition nation in 1812 conquest, as you can send aid to 2 american nations. 
But I think you could attempt the 1812 conquest as a 3 star nation even without near max tech, GB and France  are good options to get A rank rewards. GB because you get to dictate every theatre of war (America, Spain and your invasion of denmark) and France because you have OP units at the start, great gens (napoleon is OP with his ambulance) and a large  economy tip, for France, take out prussia first and don't push into Russia at the start. Expect about 60+ turns for this conquest as a coalition nation, more for a french eagle nation (unless u play as USA) as you will have to go to the americas yourself to help, USA only pushes mexico and canada back, not take them out. 
But, make sure you have healing items, flintlock pistols, art telescope etc, basically good gear on your gens before you attempt it 

Ps, I don't think I have a screenshot of this, but once my buffed davout (+20 atk with murat aura) dealt around 320+ crit dmg on a unit- yeah davout's crits are very deadly.