Struggling with Spine Fatigue During Squats – Advice Needed!

So, yesterday I increased my squat by 10kg. In 2 months, I’ve added 15–20kg, and now I can easily do 6 squats. However, I’ve noticed that it’s not my quads giving up—it’s the load on my spine. I don’t know how to explain it, but it feels similar to how your back feels after doing a set of RDLs.

My form is good, and I feel the movement in my quads, but I think my spine is weaker than my quads. Is this a thing? Should I start doing isometric holds or static holds, like loading 120% of the weight I can do for 6 reps and just standing with the bar?