Supported sitting is fine to start introducing solids. It's quite uncommon for babies to sit independently at 6 months as their core still needs time to strengthen, but as long as your baby can steadily hold their head and bring hands to mouth, you're good to wean. My HV just told me to roll up a towel and put it around baby's bum in a highchair for more support when he eats.
As long as they have head control then it’s fine x
My baby girl turned 6 months yesterday and cannot sit independently, yet I will start to wean her this weekend on purees first. Once she is better at sitting I will start to do more BLW. I have been advised by the dietician to start weaning at 6 months, as this is the prime time to introduce allergens