No vested interests here. I have no involvement with GDST at all. I really don't care about this one way or the other. My only interest in this thread is correcting untrue statements.
that the schools could still afford to stay in the pension fund
On unrestricted funds (which is where your pay and pension come from), they are running an operating deficit - £2.3M in 2019, £1.8M in 2020 (in large part due to actuarial losses on the defined benefit schemes for support staff, the main one of which is closed to new entrants). The overall operating surplus comes from restricted and endowed funds, which cannot legally be used for your pensions unless they were donated for that purpose (which is unlikely).
Whilst it is true that their reserves are just over their target level, the target they have set is significantly lower than that recommended by the Charity Commission. If the deficit for 2021 is similar to that for the last two years, the reserves will have come into the target range. In any event, the reserves are not there to be spent on pensions. They are there so that the charity can continue operating if its income dries up and to meet other unexpected events. Similarly, as pointed out previously, reducing capital spending only helps marginally.
Looking at GDST's accounts, they clearly need to make savings. That doesn't mean it has to come from pensions, but they need to make savings somewhere.
how SMT have chosen to reward themselves (generally with the connivance of take governors that they have had a role in appointing)
I'm surprised you say the governors have a role in setting SMT salaries. For a charity, they are normally set by the trustees.
When appointed, Cheryl Giovanni was on salary and pension contributions of £266,399 - about £1,500 more than her predecessor. Whilst she had a significant rise in 2020, she did not have any pay rise in 2019 and a smaller increase in 2018. Her pay has not kept up with inflation. As I have said previously, I don't know if her salary is value for money. It certainly makes her one of the highest paid charity CEOs. As far as I can tell, most of those paid more than her are CEOs of medical charities such as the Wellcome Trust.
increase in office costs (including a large marketing budget) and ‘office costs’
Support costs were reduced by £2M in 2020. Around £200k of the saving came from marketing, which is something to which you seem to object but which is increasingly necessary for independent schools. Governance costs have barely changed from 2016 to 2020.