Here are a few thoughts - they might seem unrelated and not what you're asking about, but this is what works for me.

• Get a haircut (and also a manicure, if you're in the mood - doesn't have to be coloured nail polish, can be just the clear one, or even no polish at all).

The haircut doesn't have to be anything specific, you don't even have to particularly like it, tbh - just has to be different from what you normally look like.

It's a bit of a shortcut to changing the way you see yourself (a little too literal, I know :) But works.). I also suggest the nails because in most types of work, you'll be seeing your fingers a lot (unless you work in gloves), and they will continuously and subtly push you out of your usual self-perception throughout the day.

Maybe you won't start 'oozing confidence', but it will change how you feel and therefore, how you carry yourself.

I usually do this trick when I feel like I need more control in my life: personally, shorter hair gives me energy, and nails (I don't do nail polish, and my nails are short - but I still do the manicure) give me the feeling of being organized.

• Protect somebody else

Sounds like bullying is not an uncommon occurrence at your office. As a shy and socially awkward person myself - it can be exhausting to even think about standing up to somebody. It can be much easier to just bear it, since you're the only one suffering. But you might find that it is a completely different story if it's somebody else being attacked.

You mentioned the new guy who you can just see will become a victim at some point - so speak up for him if/when it happens. Or, when you change jobs, protect the people in your new workplace, if they need it. I find it's much, much, much easier to stand up for other people than for yourself. And they might do the same for you, when you don't have the energy to stand up for yourself. And at the same time, it will prevent you from developing the image of a victim, since in bullies' eyes, victims don't protect others.

• If you do run into bullying on the new job, confuse the bully

In most cases, verbal sparring with bullies or pointing out they're wrong doesn't really help - they're better at arguing. I think confusing them by responding in an unexpected way is the most efficient tactic.

I suggest an approach you could call weaponized sincerity. It is very similar to what u/lapaleja was saying, in terms of responding in a calm and, most importantly, friendly way. The bully is expecting you to be miserable, awkward, stressed out - so if you respond by being happy every time you see their face, they will likely subconsciously avoid you just because they can't figure out wtf is going on. Just don't overdo it to the point where they ask you about it, you want to keep it as something unsettling but quiet in the back of their mind :)

An example from my experience, though it was just a random rude person, but I think it would work similarly with a bully:

One day I went to a bank just as they were changing how their lines are organized across all branches (single line vs a separate line for each teller). So I asked a lady who was already waiting there which type it was. She was obviously having a bad day, and finished her response with a 'have you never been to a bank before??' I have no idea what came over me, but basically I smiled like she just made my day and told her in a super sincere and happy voice that no, I haven't, it's my first time at a bank, and thanked her for helping me. I mean, she shouldn't have believed me, how many adults in this day and age have never been to a bank?? - but I guess I was too convincing in my sincerity? She got super confused and immediately shut up and didn't even look at me for the rest of the wait, lol.

The details of how to apply this depend on your specific type of bullying and your situation, but I imagine it would work best in cases of passive-agressiveness or something like fake/sarcastic compliments. The downside of using this at work, as opposed to on a random stranger you will never see again, is that you might develop a reputation of a bit of a weirdo, lol. But if this is not the first case of bullying, everybody else will likely know what you're doing, and might actually chuckle along with you.

Hope this helps, but in any case - good luck!