Your initial D values are really closely clustered – from 60% for initial Easy to 72% for initial Again. But that doesn’t necessarily mean something is wrong – it means that your cards don’t vary much from each other in objective difficulty.
[Sorry, this is the best screenshot I could get from Anki FSRS Visualizer – but you can plug in your parameters and click the “Difficulty” radio button to see this there.]
I strongly recommend that you do not manipulate your parameters yourself. D isn’t a value that you need to try to improve. It’s a value that FSRS uses schedule your cards more accurately.
You’re not reaching your DR (Desired Retention), so there doesn’t seem to be anything objectively wrong with your workload staying high – from an “is FSRS scheduling my cards correctly?” perspective. Obviously there are still good reasons your want a lower workload 
– but that doesn’t mean FSRS is doing something to prevent your workload from going down.
It looks like this material is challenging for you. It also looks like you don’t use Hard at all – are you a 2-button grader?
That does put you at somewhat of a disadvantage, because you’re not giving FSRS as broad a scope of data as you could. I’m not suggesting you necessarily switch to 4-button grading – it has its own downsides, and especially after years of use, that would be a hard habit for you to change. But it’s something to understand about why FSRS treats your cards as more the alike than different.
Is that your overall retention for the year? Even if it’s really that close to your DR, it might be worth bringing your DR down to 84% to give yourself a bit of breathing room.
A constant backlog is undesirable for a bunch of reasons. Trying to just “chip away” at your overdue cards often doesn’t work, because the cards you work hard to revive can get lost, and you end up needing to relearn the same cards again and again. If you aren’t already, you might want to try a plan that prioritizes keeping up with your “current” cards, and allows you to work on catching-up gradually – Adressing Backlog - #2 by Danika_Dakika.