How can I reduce my overwhelming workload?

I'm sorry to hear this - It's always a really bad feeling when you simply have way too much to do. First, I recommend checking out this very similar Q&A from another senior+ engineer who is feeling overwhelmed.

That being said, here's some thoughts from me:

• Can you check in with the other senior/staff engineers? - I wonder if they're doing okay or in a similar situation. If they're also having trouble, that's something you can bring as a greater feedback to your manager. If they have figured something out to manage everything smoothly, you can learn from them.
• Is it possible to reduce the proof of concept (POC) work? -The point of POCs is that they're a way to quickly vet a full-on product/feature idea. Here are some questions I have about your current situation:
  • Can you cut more corners with the POC work so they're faster?
  • Why aren't the POCs translating to core project work?
  • Is it possible to just stop doing them as they seem to be low impact?
• Can we prioritize away some of your tasks? - An exercise I regularly did with my manager was write down everything on my plate and stack rank them by importance. From there, we would agree on what I could deprioritize (i.e. ignore for now) and what couldn't be dropped. This conversation should involve a honest conversation about work-life balance.

I also recommend this Q&A about dealing with pressure at work.

Lastly, as mentioned in the first Q&A I linked, no job is worth your mental health and well-being overall. If your manager isn't supportive and you aren't able to correct this situation soon (within 1 month), start looking for outside opportunities aggressively and maybe just leave the company without an offer in hand.