https://www.pedall.org.uk/specialist-cycles-and-bikes/

Hello. This is an organisation here in the New Forest UK. The above link shows the bikes they use.

“PEDALL provides cycling sessions for people who face a barrier to accessing cycling on the New Forest off-road cycle network. A barrier is anything that prevents someone from being able to cycle without additional support”

It might give you an idea for the type of bike you are looking for and maybe with a picture of it you can get someone to modify a bike for you, or look to buy something similar.

Hope this helps.