I don’t get how you think this is ‘exploiting’ the issue, or how the money would be spent in advance of raising it? Or, indeed, how you think the Mayor could spend each £ more than once?
As pp have pointed out, the government have told TFL it needs to raise way more money, so funds from the current ULEZ scheme have had to be diverted there. It props up the public transport coffers, so public transport in London is way cheaper than elsewhere. As you rightly point out, the corollary of ULEZ is the need to provide adequate public transport. So it is a good use of funds to plough £ raised from the ULEX back into buses and trains. For the expanded ULEZ zone, I expect funds will be used for the scrappage scheme and the increased public transport provision that comes with the scheme.
However, this means that if the money is spent here, it is spent and can’t be used for something else. But, there will be money for poorer families and people to buy compliant vehicles and use public transport and there will be more buses on the road. How much more visible do you want the spending to be?
Green spaces etc, I agree with you and would like to see more of this. I thought I had seen something about this from the Mayor, but am not up to speed with the detail. I guess there’s a limit as to what can be done when, as there would be the money or the staff to do everything we’d like at once.