I have been, in effect, on a zero-hours contract since I retired from full-time work.

There are a number of things to consider.

If you remain an employee (for example, on a zero-hours contract) the general arrangement should be similar to now.

Another possibility would be self-employment contracting to work from time to time. The potential pitfall of this is you would be very sensible to have personal liability/professional indemnity insurance. Then you would have to consider who pays for it.

"Volunteer" and "'semi-professional'" suggest less than the going rate.

Lastly, don't forget car insurance. Nowadays it seems to be social, domestic, and pleasure; and commuting. You may wish to consider how your new arrangements might affect that.

Adding business travel to a standard insurance policy is remarkably cheap.  I recently had to add it as I was needed at another office of my employer, so no more commuting to a single place of work.  It cost me £1.80 extra when renewing my policy.

Adding business travel to a standard insurance policy is remarkably cheap.  I recently had to add it as I was needed at another office of my employer, so no more commuting to a single place of work.  It cost me £1.80 extra when renewing my policy.