Low back arch and neutral spine?

Forgive me for not searching better if this has been covered. I have had a history of low back trouble (SI joint) off and on since I was 14. I believe this has been the cause of hip flexor, quad, and hamstring problems.
Often times, and lately, when I do a lift like back squats or RDL I get a sharp pain in my low back on the right side - I believe it is the SI joint again. The pain happens only at the top of the lift.
For example, on the squat, after I have completed a rep and am just about to fully extend at the hip, the pain happens.
I can get a pretty good curve on my lower spine (butt back arch). At the bottom of a full squat the extreme arch goes away - as I understand it, this would be a neutral spine position - and I head back up.
Some things that help are wearing a belt or doing some back extensions on the roman chair between sets.
Is is wrong to fully extend the hips at the top of movements like RDL and squat. Should I just keep a little bend at the hip? I am a novice dead lifter, but I have seen people really extend the hips forward at the top of the deadlift. This seems like a great way to fully activate the glutes.
Let me know what you guys think.

Extend the hips forwards? When somebody tucks their hips forwards, that is flexion and not extension. So, do you mean tucking the lower hip forwards? Or do you mean tucking the lower hip backwards?

Sorry I am not good with the descriptions. I think my problem is that I am losing the lordotic (?) arch in my lower back at the bottom of the lift and when I get to the top I try to get it back. This is when the pain occurs. Thanks for trying to help this dork (me).

In full squats it it impossible to not loose the lordotic arch in your lower back. (unless you are a mutant.) The arch reverses at bottom of full squats in my experiance and I haven’t seen differant in anyone else. Their is some degree of this even in parallel squats.

When you get to the top of the movement you should be in a neutral standing position. (As much as what supporting a heavy bar behind neck will allow.)

Therefor, once you get to the top of movement, no need to stick your butt out further. If you do stick your butt out further, in a standing position, it’s just extra unnessesary work for lower part of lower back creating an un-natural position. It is not xtra work for the glutes, as your feet will be directly under c.o.g in a standing position.
and niether would you need xtra work for the glutes in the standing position once you have got to the top of the movement.

When you get to the standing position I assume the lordotic arch is there.

Also; Chiropractors are the best people at physically treating spine and back problems.
But sort out your lift tech first.