The spinal erectors have the longest recovery time of any muscle group. The deadlift and squat both tax the low back to significant degrees. And if you're also doing things like front squats and stiff-legged deadlifts or farmer's walks or... well you get the point. There's literally no reason to tack on direct low-back work like hyperextensions and such.
In fact, doing such work could be the very issue with the deadlift plateau you're experiencing. This is why I suggest squatting and deadlifting on the same training day during the week. It gives the low back ample recovery time during the rest of the week.
Do This Instead
Build your upper back, lats, and hams. Show me a guy with a big deadlift and I'll show you a dude that can chin and row big too. Sure, there will be some really fat guys that can pull big and not do chins, but the average dude will need to build strength in the entire upper back if he wants to deadlift big weights.
What lifts should you use for your lats and back? Pick ones that allow for a high degree of progressive overload and ones you really enjoy doing. It's up to you to figure these things out through experience.
As for hamstrings, I went with the stiff-legged deadlift (from a big deficit) and the good morning during my powerlifting career.
Stiff Leg Deadlift from Deficit
I trained these with completely different modalities. I found that the stiff-leg deads were something I could push the loading on, but I kept the good mornings light and focused on the stretch.
No other lift built my posterior chain like stiff-leg deads from a 4-inch deficit. From the upper back to erectors to hamstrings, this was my "go to" for total posterior chain smashing. I eventually worked up to over 600 pounds for reps on these.
With good mornings I kept the loading in the 185-225 pound range, even when my deadlift was consistently over 700. That's the contrast in how you should be approaching loading if you choose to include these two lifts as part of your deadlift program.