Firstly, do not get confused by the statement about FISOs being a 'step up' from A/G operators. FISOs provide a different service to A/G operators - the level of service provided at each airfield is normally fixed and not determined by the qualification of the member of staff sat in the chair at any given time.
That is rather misleading on two counts. You will sometimes find that an FIS unit will provide AGCS at quiet periods, or late evening, or if a FISO is not available, so just because it is nominally a FIS unit does not mean that someone with an AGCS certificate would be excluded
As I said - it's not
normally determined by the level of qualification. Current SRG guidelines are, I understand, that units should endeavour to provide a fixed level of service where possible.
The point I was making however is that it's not some kind of hierarchical structure where a A/G operator is 'outranked' by a FISO each provides a distinct and separate type of service. The OP read as if the thought was that A/G operators may be volunteers and FISOs full time.
obtaining an AGCS certificate could obviously be a first step to better things
Agreed. But bear in mind that whilst being an A/G operator will probably look very good on a CV of anyone looking to do ATC as a career, there is no dispensation from the licensing path to become an ATCO as a result of previous A/G or FISO experience (there used to be for FISO's until the CAA pulled the plug on that (without informing the already qualified FISOs) a few years ago!
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