Your perception of the 5153 is fascinating because it is absolutely not more casual than the 5296 or 5227. The 5153 is by far and away a classic dress watch, that is very formal, and more appropriate for formal occasions than the other mentioned calatravas. The 5296 can be dressed up or down, particularly the sector dial variant which is in no way boring. The sizes of the 5296 and 5227 enable it to be worn casually with jeans and a T-shirt if you so choose.
The 5153 is as formal as it gets in terms of Patek. Definitely not a casual watch and putting it onto a non-dress suede or whatever strap will be gauche and inappropriate for such a timepiece. Doing so will sure draw the ire of actual real gen Patek owners who see you and know that such a watch isn’t meant to be worn casually and while it is FINE to dress it down casually, putting an aftermarket suede strap is just sacrigligeous. Most importantly, it’ll draw attention to the watch by gen owners who are well positioned to immediately notice it’s a fake from 6 feet away because it has so many tells. Everything you mentioned about the casualness of 5153 is actually more appropriate on the 5296, but absolutely not the 5153, which is meant to be worn to the opera.
Whilst I agree that the 5296 sector dials are absolutely the more casual watch out of the three that we are talking about, and yes I do like that model as well but I find the dial to be a little bit too cluttered for a sector dial watch. But the 5227 and regular dial 5296 ones are definitely "more for the operas." Both watches are slimmer than the 5153 and cleaner, more simple lines that work very well with a dress shirt cuff.
The 5153, for me, isn't as formal as it gets in terms of Patek. I would say the 5227, 5296, 5196, and the hobnail bezel ones (especially the 5120 which is by far the most formal as it does not have a seconds hand) are the ones that are the most formal ones you could get from Patek's modern lineup. The 5153 has an officer's caseback and screws instead of spring bars, which don't connote the formality of the model. When it comes to the strap choice, how is dressing down a dressier piece considered gauche? I've seen pieces such as the 5078, Tank Cintree, etc, on tan Hermes straps and they look great. It is in my opinion that dressier watches worn casually with aftermarket straps are great. But hey, each to their own. I like smaller watches on straps, but can't for the life of me wear black alligator straps, nor do I wear a suit to warrant one.
My inspiration for this purchase and strap combo is from aCollectedMan's Simplicity photo: