Had it not been for Covid First would've already been at a push, just look at the amount of Euro 3 buses still in the Glasgow fleet. The 2020/21 order is basically cancelled given that it's likely being delayed until next year, so that'll leave just the 2021/22 and 2022/23 financial years for new vehicles. There's still over 200 Euro 3 vehicles in the fleet not including all the Euro 4 B9TLs, some of which won't be eligible for funding due to their age. In short they'll need somewhere between 225 and 250 new vehicles, i just can't see that happening in a two year period, granted the 2018/19 order was for 150 vehicles but it has always been said that subsequent orders wouldn't be as big.
That's not including the fact that FSE will need LEZ compliant vehicles for the X10/X10A/X35/X36/X37 (assuming the X35 is extended back to Glasgow), currently the only LEZ compliant vehicles Midland have are the E400MMCs used on the Stirling University route. As for Aberdeen... basically the entire fleet will need replaced on converted as there's only 7 Euro 6 vehicles in the current fleet.
I wouldn't get your hopes up regarding Trains, Hydrogen trains are a technology not currently developed enough and clearance issues on the West Highland Line will likely see the 156s last a good while yet. Taxis should be considered private cars and banned from bus only zones, they don't help reduce emissions or congestion. If anything i'd favour electric/Hydrogen powered cars being allowed in bus lanes over taxis.