Fed up of being used

I think an interview is to see what sort of person you are and if you would fit in as well as if you are qualified enough unfortunately you choose to use it as a free professional advice session which may have been overwhelming and also seen as very critical of the organisation
It was your choice to offer them your free very professional advice
A big shame for you but remember next time keep your main knowledge back until you get the post
Good luck for any future applications

Relooking at your OP AliceS maybe they took your feedback as too critical. If those interviewing you were personally invested in the things for which you offered constructive advice & may have taken umbrage. Or was there a question in the process asking for your opinions on website etc?

It sounds as though you were overqualified for a volunteer position.

Maybe the Director wanted a volunteer who be quiet and nod a lot?

I think anyone who engages in volunteer work sooner ot later comes up against a group, that ask for help that they do not really want, or cannot use.

I have been involved with a number of organisations over the years and in most cases the work has been fulfilling and enjoyable, but I have also had disappointments as you have had, with charitie that are overjoyed when you contact them, very keen and then either cannot or do not follow through.

Keep trying, there is a charity out there that is desperate for the skills you offer.

Maybe their style of management & ethos did not align with yours...might be a lucky escape.

Try looking at the Inspiring Governance website. Schools are always on the hunt for governors & trustees with skills, knowledge & time to give.

I d put it behind you and forget about it ….their loss
Perhaps you offered too much advise for an interview and they felt overwhelm by your knowledge

Why not try some more ‘simple’ volunteering where you ll have some fun Think what you enjoy, apart from academia,
Good luck

I'm sorry for your poor experience, obviously one organisation that should have jumped at getting you on board and didn't. But there are lots of other organisations, keep going until you find one that is a good fit.