Is there a reason to get this if you have CK2 and 90% of its add-ons?
At this stage I would say no, unless you are fed up with CK2 and want something looking and playing slightly differently, yet being almost identical at the same time.
The good things about CK3:
- the hooks system (now you can blackmail other characters and make them do your bidding if you know their secrets, works sort of similarly to favors in CK2)
- you can actually be a tyrant now without everyone revolting against you instantly, when you do cruel actions you accumulate something like dread (dunno what it's called in the game) and if it's high enough it prevents other characters from taking actions against you.
- religions are much more detailed, you can see their doctrines (for example if the religion allows only men to be priests), which traits are seen as virtues and attitudes toward certain things like homosexuality, adultery, deviancy. You can reform a religion and customize all of those things, but this has its downsides. You can reform a religion to be a complete opposite of what it originally used to be without triggering some massive civil war, making it very unrealistic.
- the map looks great when being zoomed out.
- character models are not that bad either.
- you can see baronies on the world map.
- great loading screens.
- the stress system (doing actions that go against the traits of your character accumulate it)
- it's possible to create a cadet branch of your dynasty.
The bad:
- horrendous, atrocious UI developed for phones and tablets.
- lack of latin names of certain things like laws etc.
- you can only declare war for a single title, you can't press multiple claims like in CK2.
- only three government types (expect more with dlcs).
- lack of certain filters in character search options
- lack of certain map types.
- lack of societies and secret societies (will probably appear in a dlc).
- you can't secretly follow a religion (will probably appear in a dlc).
- only two starting dates. (I don't view it as something that negative, because I only used the earliest ones in CK2)
- no option to create a randomly generated world like in one of the CK2 dlcs.
- smaller amount of events (will improve with future dlcs)
- no plagues and epidemics (will probably appear in a dlc).
- no bloodlines (will probably appear in a dlc).
There are likely many things that I missed, but this will give you a decent picture. If I were you, I would wait for it to gain more content through dlcs and buy it on sale. I don't think there is a reason to buy it right now at full price, especially if you own CK2 with most dlcs.