You always lose all 5 vassals when you pass a reform, it has been that way for years.
As fir advice, I have played Aztecs many times so I have some tips. The most important thing to be aware of is who to attack or not. As the Aztecs, you can pretty early take on 3 countries at once alone if you play it right. Use your level 3 fort, position your army to instantly stackwipe one enemy army and take their provinces, and make sure to always have as big an army as possible to allow swift victories. Taking the discipline bonus for the first reform helps a lot with this. Something to keep in mind when picking wars is also that almost all vassals around you are disloyal, which allows you to occupy the overlords without resistance, and then take on the vassals afterwords for some very easy wars.
When you win wars, you obviously can't take too much land because of doom, but there are still some things to consider: First off, always take any provinces with forts, that makes it much easier to control the land and fight numerous nations later on. Second, take provinces so that you start bordering as many new nations as possible, this way you have more targets later on. But, make sure to never wipe out any nation, because you will need them as potential vassals. Lastly, if you after a war has the option to have more than 5 vassals, it can be worth it to fully conquer some nations (but don't core them) so you only have 5 vassals. Then pass the reform, and afterwards release those nations as subjects again. This method gives a lot of aggressive expansion though so be careful.
About doom management, I find that ca. 6 doom yearly is easily managable, so taking provinces is fine if you stay around that amount. Also remember to sacrafice vassal heirs/rulers on cooldown. In addition, if you have very high doom, you can farm doom by stackwiping a nations army but leaving their provinces, wiping out every regiment as they spawn.
The most difficult thing when playing Aztecs is manpower IMO, especially after the mercenary change. To manage this is difficult, but ideally try stackwiping evry army to avoid losses. Taking the discipline reform first, along with sumptuary laws event modifier and advisor should allow you to stackwipe pretty much anyone in central America if they don't get reinforcements. Also try to always stay on top with military technology. Use mercs if your manpower is low, momentum is key. If you get bogged down your doom will start rising, and a stable balance of power is the worst-case scenario for the Aztecs. Thankfully, with nations constantly becoming your vassals and independent over and over, there should be lots of wars to exploit for your advantage.
About monarch points, you won't really need them before the europeans arrive, since you get all the tech for free. A lot of people suggest getting admin tech 5 for more early colonization, but I usually spend all my admin and diplo points on development. That gives you a lot of money for mercs and advisors, but it might not be optimal.
