I worked for a horrendous woman who insisted I go full time, out of some kind of autocratic spite because she hated women with children.
I had only ever been 4 days per week. I had applied for the job 5 years prior and negotiated the 4 days from the outset.
I could have taken it via union/lawyers, but its really not my style, so instead I just applied for another internal job and handed in my notice.
As I was working my notice period, they started to realise how much I got done in 4 days and the panic mounted.
They ended up having to split my job into about 4 different new roles, all of which were difficult to fill. The most difficult of the 4 roles, managing the finance team, was an absolute car crash. One after anotger person went into the role and either left or went off sick with stress, then left.
The easiest of the jobs was essentially a "knowledge and expertise" job, that moved teams, paid the same as my old job, for at most 30% of what I used to do. The new department offered me that job on my old salary, on 3 days per week. 🤣🤣
I luxuriated in working for a nicer team, on almost as much money, doing much less work, with less stress.
I'd be tempted to do exactly what you've suggested. Apply for other internal roles!! Someone will be more than happy to give you a better deal.