@gamersrights69 Can you be more specific? All Ped models, game ones, all ped addons, specific addons?

Menyoo will behave inconsistently with certain models because authors, for whatever reasons, don't follow conventions.
For example slot 4 is meant for legs but in some mods it is for glasses. Try using Apply or Apply Model - they can give different results. The first usually works, the second forces the correct model/outfit combo.

Many vehicles are badly flawed as well. I have about 3 or 4 vehicle addons that spawn missing critical parts like the frame or engine because of how they were created. You then need to play with the spoiler or the chassis or whatever to restore them. I do this through modkits (C#) but it can be done in Menyoo.

In passing, the issues you describe, have nothing to do with your installation, no scripthookv scripthookvdotnet, dinput8.dll, openiv.asi,, etc.
And they don't appear to be due to dlclist.xml - you make the slightest mistake in dlclist.xml and your model will not load. And dlclist has nothing to do with outfits.

As a63nt-5m1th suggested, first step is to install current version of Menyoo or a new trainer. I can tell you from my own experience, with approximately 150 vehicle addons and about 100 to 200 ped addons that Menyoo is rock solid. Only issues I've had are with bad models. I use Menyoo to customize my peds and vehicles and then I take the values, either writing them down or from a saved xml file, and convert them to C#.

For disappearing cars - that does happen - there is a mod called addon spawner that will allow you to spawn with persistence.
I know GTA5 is inconsistent with this. I can save a game with some cars for example and then turn off autosave and play for many sessions with the cars there on load and then on a random session, for no reason (i.e. the game engine), one or all of them are gone.