They had an entire battle scene in Casterly Rock that didn't even happen. Time and money was not what they were short on, it was an explicit decision on what to focus: making Tyrion sound smart and have a scene where the Unsullied look badass and fighting a force that others would lose against was completely unneeded given it ends up with the plan being a bust and the Unsullied looking cool... by killing a few hundred men at most, only to lose to Euron's teleporting fleet.
They could have cut to just show the Unsullied take over a seemingly empty Casterly Rock and then have Euron's CGI fleet blast their CGI fleet for a few seconds, and then just show some minimal effort with Highgarden.
Hell, if they really didn't want to show the battle of Highgarden, they could just show Jaime arrive and then reword some of the dialogue when he meets with Olenna to convey the time and effort that passed. Instead, they had Cersei give a maximum time of 14 days from when Jaime leaves with the forces from Casterly Rock and until the gold is in King's Landing. Unless it takes a day of travel with wagons from HG to KL, there was no siege, no preparations, not even constructing basic ladders or anything: Jaime just shows up and the doors aren't even locked, and by all accounts HG was defended by 30 dudes or something. The rest of the might of the Reach apperently does not exist: only House Tarly seems to have any troops, and not that many given only 10,000 Lannister and Tarly troops were present for the Field of Fire 2.0.
The Reach went from being the biggest military power to less relevant than the Riverlands, with no explanation because D&D need to take away Dany's advantages (which they only gifted her with them in the last episode of last season) to make it look like the conclusion of this war is ever in doubt and to drag it for an entire season.
We could have had some lines thrown about the Reach Lords turning to Cersei other than just Tarly, or how the forces can't reach HG in time and once Olenna is dead, they turn to Tarly, who was made the new Lord Paramount of the Reach or something. It wouldn't be hard, or even demand many new scenes or lines of dialogue to make this more acceptable. Just decent writing.