My impression is Cassandra was called away alot by family and acquitances for assistance; the birth of a child or an illness...Cass. was away at Godmersham, Edward's home where the arrival of a sixth baby made her presence desirable when Mr and Mrs Austen decided to move to Bath.

Cassandra remained with the Lloyds at Ibthorpe a few weeks while Jane and her mother travelled onto Bath; I don't know if anything urgent kept Cass. away.. but the Austens were still looking for a suitable house in Bath while they still lived with the Leigh-Perrots.
Even if there was no urgency at the Lloyds, possibly Cass. wasn't able to travel to Bath alone.

I agree, Jane's letters reveal pain, sadness and depression.
I wish she had the comforting presence of Cass. who may've felt as frustrated as she did.
'Cassandra destroyed several letters Jane wrote to her immediately after hearing her parents' decision, which suggests they made her uncomfortable, too full of raw feeling and even anger'. (Tomalin, p170-171)
It was a time of great uncertainty; neither Jane & Cassandra had any control over their sitation and as the unmarried daughters, never appear to be consulted by family members.