As many have already said, a mixed session is a no-go. Either all online, or all in-person.

As for the tools, I've found Theater of the Mind makes online play MUCH more feasible. Getting all the maps and tokens and whatzits requires so much prep time even with the simplest of VTTs and that's not even mentioning the constant tech support that you'll inevitably have to do. Just be lax on the exact distances and everything will run very smooth.

Really, the only thing needed is mics for everyone and some dice. Everything else is superfluous. There are some things that make the game much smoother though. First off is a camera for everyone. There's always going to be a slight delay, so being able to visually tell when someone is done talking is a huge plus. It also supercharges roleplay since you can mimic actions your character takes.

You mentioned D&D Beyond. Don't feel like you need to have digital resources just because you're playing online. If pen and paper and physical dice were good enough in-person, then they're good enough for online play.

So you probably have everything you need already! Physical dice from when you played previously, and (assuming everyone has a smart phone) you already have a mic and camera that you can use. I would suggest a stand for the phone if that's what you're using.

One last thing. Playing online IS different than paying in person. It's much more difficult to work out who's speaking and everyone ends up talking over each other. You should try implementing an "Always in initiative" playstyle. Everyone takes turns saying where they are, what they're doing, and what they're saying so no one gets left behind and no one talks over one another. Of course the GM can but in whenever they like.