My thoughts on diamond formations in FM:
The wide diamond is viable in FM22. You will concede plenty of goals, but you can generate a lot of offense with four wide players, two strikers, and an AMC on the pitch. Even if you start with a narrow diamond, it's good to have positional flexibility to throw a wide diamond out there if needed.
In a wide diamond, I would prefer to use conventional wingers (or maybe defensive wingers) plus two inverted wingbacks to bulk up the central midfield area. Inverted wingbacks rival CM(a) for role of the year, just with less publicity.
A narrow diamond is also viable, even with the super-skinny default in FM22. I give my central midfielders "stay wider" instructions, regardless of which role I give them.
In all diamonds, I find the forwards produce best when used as AF(a). Other options can work, but you have to be careful not to put all of your eggs in one striker basket. One of the major benefits of a diamond formation in FM is that you only need one of your two strikers to be on form for you to succeed. In a 4-2-3-1 or 4-5-1 formation, a struggling striker can sink you very quickly. Furthermore, using two attack roles means they'll often be in position to play each other in, and they tend to be more aggressive with their pressing.
The AMC is open for debate. I don't love the AP role in this spot and prefer to move the playmaking job back to DMC or central midfield. I want my AMC to be a third goalscoring option, so he has to have decent finishing, good off the ball, and the attributes necessary to evade marking (agility, off the ball, acceleration). He should be able to convert those loose balls in the box into goals and work one-twos with the forwards. I wouldn't waste this spot on a lazy old player.
The DMC is where I'll use a DLP, HB, or AM. I would prefer the supposed conservativism of the HB role, but I often see them getting too far forward in the match engine, so I just stick with a DLP and put my best playmaker back there. It's a very easy role to fill in the transfer market, and I find dozens of suitable 30+ year olds available for cheap each season.
I always give my fullbacks a wingback role (inverted, complete, or conventional). FB just doesn't seem to do enough or position aggressively enough. I tinker with the role and duty based on match situation, going much more aggressive if I need a goal. I won't use a diamond unless I have four playable wingbacks on the team because I don't want to be stuck with a defensive fullback or converted CD in that spot.
I much prefer the shapes of the alternative diamonds, 4-1-3-2 or 4-3-1-2, but they don't always succeed in the FM World. In FM22, I have found the 4-3-1-2 to be very leaky defensively. The 4-1-3-2 is the opposite, but it can be very tame on offense unless you get a beast to put in a CM(a) role in the middle and have excellent attacking wingbacks to feed your forwards. Don't even bother with these formations if your wingbacks are duds.
Finally, I love the pressing I can get with two forwards and an AMC. Surprisingly, it seems to work much better than AML-ST-AMR does. AI teams LOVE passing in a triangle between GK and two of their defenders. They'll do this for half the match if you let them. The diamond formations disrupt this a bit, which is a lot of the reason why I prefer watching them play out in FM.