I am not a mum but I am a child psychologist, and thought I would give you an opinion from a child development point of view.
I think its great that you are so concerned about this and speaks volumes about how you are trying to create a safe and secure environment for your child to develop in.

You're right, the ideal is that children do have a stable routines in environment to help them develop emotional stability, as well as within a range of other areas. It helps children understand their environment and predict what will happen next.
There a range of things to take into when considering a child's adaptability to routines account too, including:
1) age of the child
2) emotional ability of the child
3) developmental ability of the child
4) overall environment

Bearing these factors in mind, should the overall environment of the child be reasonably stable and there are no developmental concerns, then most children are reasonably resilient to changes in their routines. I would advocate for trying to create as much structure as possible and therefore the Mon-wednesday, Thurs&friday structure seems most appealing. Furthermore, attempt to create 'mini-routines' that help the child predict what is coming next. It is also important to bear in mind that the child may need some extra 'TLC' doing the transition period, particularly at the times of change. Also, should your child be old enough and with a good ability to understand I would suggest actively involving them with the planning and preparation around the changes, such as setting up a weekly calendar with them, or preparing them the night before.

I hope this helps!

With best wishes,
Stephanie

Dr Stephanie Satariano
Child and Educational Psychologist
V.I.G. accredited Guider