I feel your pain! I got a family toilet seat with a step in front and devised a toilet desensitisation plan for over a 2 week period before my dd started reception. I did it really gradually with v low pressure because I knew if i forced it she would freak out more.
Day 1-3 potty placed next to toilet which was always set up with the kids seat and step ready, and let her use the potty as normal but talked about the new family seat “we have a new big girl toilet seat for when you’re ready!”
day 3-5 wow look at how the new toilet seat has a big girl seat on it! It will fit you perfectly! Do you want to see how it lifts up and down?
day 5 from tomorrow you can start practicing little sits on your big girl seat! Yes of course wees and poos still on potty if you want to, there’s no rush!
(day 5 she had the smallest sit “that’s amazing! Did it feel comfy??”) from tomorrow we can do practice sits at bedtime to help you get more confident!
day 6-7 help her sit - even a little sit counts, loads of praise for making small steps to success
day 7-9 build on that - sit more often and float the idea of trying a wee “when she’s ready”
day 9 onwards encouraged to try for a wee…
by this point I think she’s gained the confidence to do it by herself and started using the big girl toilet seat and was so proud of herself
to add to the motivation she got a new Gabbys dollhouse figure every day for doing her best -accompanied by a low pressure encouraging written note in the voice of the character (i used ChatGPT to make these and she loved seeing the surprise note and toy each morning)she has kept these notes, she thought they were real dollhouse deliveries, it’s quite sweet.
she got a big toy at the end for mastering it.
i was worried about her refusing to go to the toilet at school - she is a determined girl and would hold it in all day in defiance! I didn’t want her getting UTIs/constipation or having accidents and being picked on.
Sorry for the long post but this worked for me!