You didn't mention tests. How do you know their 'progress' is actually correct or even good?
I'm presuming the 'several other developers' are younger than you, hence your concern about age. Without some measuring stick to know if what they're doing is correct, you can't tell if they're just doing more work or actually doing it correctly.
> How do I find even a single chunk of manageable work to break off.
How is anyone else finding work to break off? Is this a free for all, or are there any planning sessions to identify things? Without that, you might be overwriting or reworking something someone else already did.
You may want to ask for some pairing with someone else who you think is doing this better/faster than you, and just ask to work with them for a couple hours.
Talking through the problems with someone else might help turn a corner with respect to mental models. How are they reasoning about current code? How are they tracing through the paths? Someone else deep in this problem may be able to come up with language or examples that resonate with you in a way that random HN people can't.