How to obtain entity/spawner data from a server

On a vanilla minecraft server I play on there are a lot of different custom mobs and corresponding custom spawners. I'd like to make a list of the stats for each mob, but this is difficult to obtain - getting killed by one only shows me their weapon and clientside health bar mods only give a vague indication of armor points at best.

Is there a way to obtain more detailed information from an entity (or spawner) on a server from clientside only? I'm mainly interested in exact gear stats and active potion effects, though I'd also like riding entity stats (since some mobs have splash potions riding them that drop when they die, or have mounts)

You can press F3+I while looking at an entity to copy its data. If you have command access, this will give you the summon command for the entity, with all its NBT data (be careful when summoning duplicate entities with the same UUID). But if you don't have command access, this only gives you its type and exact position.

If you want more information, then you would need to use a custom client. I don't know any that directly shows all information about an entity, but for example there is a "world downloader" mod that allows you to create a local copy of a server. That should include all the information your client can get about a world.

(Also note that Tags and Passengers in NBT work weirdly, so you might get different results depending on what you use to get that information.)