What are these games like btw? I didn't realise there were so many entries
To add on Grav's notes: The startoff being originall and eroge is also a reason, why Prelude is somewhat fairly diffrent than the rest. The sex scenes are now gone, but the (sometimes a lil bit feeling forced to dumb) situations leading to them are still there and now not making particular much sense. The one where it actually made sense (and more or less is actually still needed to be happening, plot linking wise) got its scenes skipped too, and not with particuliarly a whole lot ambigruous implication of it happening. As is, you have to swallow that it apparatenly happend somewhen somehow off screen now.
Deception and Truth came 10 years later, they are also fairly diffrent in tone and narrative and pacing, with having a diffrent main cast, diffrent initial setting and all (but same world and plots are linked.) As such Deception is actually quite beginner friendly, though knowing Prelude you get some of the references much more.
Truth being more like a Part 2 of Deception is definitely fairly true. Part 2 leads seamlessly into Part 3, unlike Prelude, which take place some 2 decades (at least) before Deception.
Zan being a Spin off is not entirely untrue, but I would rather call at a badly abridged game focused summary version of Deception. It doesn't add anything to the plot as you usually would think a spin off would do.
Basically it probably just wanted to give those, who wanted more game parts some more game experience and didn't even bother to integrate the VN parts again. (Most are just summarized like "a fierce xyz happened and suddenly they's in the next plot stage.) But then, they also didn't skip it completely, so the scenes which stayed 1:1 now feel like a total pace breaker. (And ugly to look at, too, since instead of the artspreads you get uncanny 3D faces trying to replicate the,.)
Other than that it is wise to avoid anything about ZAN's promotional material, if you want to avoid getting spoiled. The ending of Deception is literally the the first 5 seconds of Zan's opening and they placed scenes the biggest plot direction changing segment right into PV's.
On a P.S. note, anyone who wants to get into it, but doesn't want to deal with that Prelude being OOP or waiting for steam - it's plenty fine to just watch Anime S1 (with Funi) and jump to the Masks games. (There is just a very little scene they left out which would have linked it better to Deception, but you can still put 1 and 1 togethter yourself.)
S1 in generall is a pretty faithful adaption and also counts as some sort of cult classic anime wise.
(Unfotunately not even remotely the case for S2, that's more of a borked adaption than not. But it still did its job of getting me hooked into the franchise, so I guess its existence has had a point.)