I am currently writing a proposal for my company (small consultancy) on a method to create a successful job share scheme to avoid this exact pitfall of part time working.
I would absolutely love the thoughts of others... have you ever worked in a managerial/senior job share that has worked? Do you have ideas for making a scheme work?
The only job share schemes I have observed put the onus on the employee to find someone who does a similar job who has similar (or opposite) working requirements and initiate the process. That's a needle in a haystack in itself but chuck in the fact that it then leaves very little thought for the comparability of those two people then it's almost doomed from the beginning.
This leaves people who are working part time disillusioned and feeling stuck but also leaves a huge, highly experienced, extremely driven section of potential workforce untapped.
I have been successful working 3 days a week but because my company is very new it has been in a very unstructured way. I earn more money, WFH and have loads of flexibility. I feel very fortunate and consequently want to use my position to open similar opportunities to other women.
I would say that it helps to reframe it. Break down a full time job... You have tea breaks/lunch breaks, probably participate in unnecessary meetings. If you work in an office job then it's not unlikely you do a bit of personal admin on company time, etc.
When you work 3 days most of that disappears and you spend the time much more focussed. I frequently reassure my clients (who always want 5 days) that they are not losing as much time as they think. So sell yourself and sell WHY you CAN do the job in less time.
(If you happen to work as a PM, PMO or in change/transformation then PM me).