3rd person caveman/toddler speech

Grog lift big thing!

My suggestion is to avoid any cringey back story. Seems like all characters have some edgy drama they center on. Instead, have a really boring backstory like you're a retiree who is just looking to pass the time. Something you can kinda chuckle at to break the tension with having to roleplay.

Or, if you have to have an elaborate backstory here is one I've used: your character's mind broke after whatever happened (like old age) and they believe they have made a pact granting them warlock powers. They don't have those powers and are just going batty, so just work it out with the dm where you "cast" hexblade but just do a normal attack. So you're just a fighter really but you'll be pretending you aren't with the other players.

Lots of great tips here, but one that works pretty well for my group, is to think of myself less as an actor playing a character, and more like a narrator. Which is to say, I'll often describe the actions and/or thought process of my character before saying what I think they'll say (in my normal voice).

This is easiest for mages since cantrips like Prestidigitation become powerful roleplaying tools, but even something like holding a Fire Bolt in the air over the person you're trying to intimidate can have an interesting affect on the mood of the scene.

I've only done a little time as a DM, but myself and all of my players were diversly nuerodivergent.

Some of them just did not have a performative bone in their body and I considered making or buying some kind of button that simply lights up while holding it down, as a visual indicator that they're speaking as their character.

Probably not for everyone, but seemed helpful to a couple of people.

If you're using cameras, you could just use a prop to visually separate you from your character. Glasses/monocle, cigarette/pipe, an eyepatch you can flip up or down, etc.

The act of deploying the prop would also help as a reminder to use whatever affectations you might want to when speaking too. When you want to go back to speaking OOC, you can just put the prop down.

There's always a Vow of Silence, more facial expressions and gesticulating than funny voices.

Overconfident can be a lot of fun to play, especially when paired with a flaw, like mispronouncing, using words wrong or making words up. Like Dr. Teeth from the Muppets, for example.

Generosity. Unyielding unflinching generosity. Giving the other NPCs and PCs the attention they need.

You can also look for ideas in books like Hillfolk (has ideas on how a “dramatic pole” can help, i.e. being torn between two conflicting values like home life vs work) or Play Unsafe (such as playing with status, being a noble or a servant). Play Unsafe also has the wonderful tip to not be afraid to be boring or obvious when improvising, to say the first thing that comes to mind as opposed to trying to force a creative or unique idea, because what you think is gonna be boring may well be super interesting to the other players (and when it's not, it's at least something basic and fundamental they can easily build on).

You can also collaborate with another player for a type of relationship with their character like being their body guard, religious follower, sibling, servant, spouse, teacher, or student. (Only if they’re into it, of course!) Some storytellers over the years have used extremely shallow and two-dimensional characters to great effect over the years simply by having a cast of characters collide with each other—people meeting other people is great story fuel.

What kind of character are you playing currently?

I completely feel you there, and I genuinely wonder if another game system might help to practice RP via storytelling focus vs. D&D's notorious crunch, ya know? Maybe a one-shot or short-arc in DOGS, Blades In The Dark, etc.?

Try doing a sing-song voice for your character (presuming your natural voice is not already sing-songy)

Have you tried in person? I honestly have never tried playing online, but I've heard that the energy / enjoyability of in person is far better.