The lap belt and 5th (and 6th if you deem necessary) points should
feel like a snug climbing harness holding your pelvis down. The
shoulder straps then have something decent to attach to, and don't
need to be tight enough to play bass guitar on.
Having the shoulder strap points below shoulder height and too tight
will only serve to decrease pilot comfort.
The design of the seat pan doesn't matter. A crotch strap system is
needed in all gliders. Don't let the lap belt ride up.
If the belts slip because they're old, don't just keep tightening
them. Get them rebuilt (new webbing) or buy new ones.
Jim