The timelines for the ships this season bothers me less than the 1000 ships being built (although I have a few ways to logic that away, namely they didn't actually build 1000...) or how the fuck Euron got his fleet in and out of King's Landing multiple times, when that should mean going right past Dragonstone each time.
I have to assume we're not done with Dorne. Tyrells? Sure, over. They've been in the limelight for the last few seasons, and the explosion at the sept was really their end. Losing Highgarden was just the cherry on top to clear out the last loose thread with them. Martells will clearly come into play with Sam and his father. Dorne still has an army, one that hates the Lannisters, and is still itching for a fight. It just needs a way to King's Landing and a leader. A rescue at King's Landing could shift that stuff a bit.
Part of the trickiness is that the show is coy with when timelines are actually happening relative to one another and how long the gaps between scenes are. Eg the Dany and Euron scenes might not have happened in the order shown. This episode was really heavy on large movements - Jon from Winterfell to Dragonstone (though he wasn't leading an army, so could have moved quick), Euron from King's Landing to Casterly Rock, Jaime from King's Landing to Highgarden (presuming he sent an order via raven for the Lannister army to head south). Those are all pretty big trips, so the episode likely would have been 1-2 months of real time, easily.
For Euron, it's not clear if he was in King's Landing swearing to Cersei before or after Dany took Dragonstone. For the sake of timeline I'll assume he had already sworm fealty and was hanging out somewhere in the ocean when Dany reached Dragonstone.
So sequence would have been:
Greyworm/Unusllied depart from Dragonstone sometime around when Yara does for Dorne. I'd guess Greyworm left first due to distance to target.
Between Dragonstone and Dorne, Yara and co are overrun by Euron (who sailed out of King's Landing or already at sea) and captured. Depending on how early in that trip that occurred, Euron's trip back to Kings Landing could be pretty short.
Euron returns to King's Landing and presents his prize, and almost immediately turns around and sails for Casterly Rock. Jaime was at the throne room when Euron presented his gifts, so Jaime would have departed for Highgarden around when Euron leaves.
Greyworm and his army arrive after a pretty long trip to take Casterly Rock (Dragonstone to Casterly Rock is pretty similar in distance as Mereen to Dragonstone) quite quickly - the actual assault was short, but they may have not attacked immediately.
Euron arrives and wipes out Greyworm's moored fleet. Not surprising - Unsullied aren't sailors, they'd suck in an open water battle. They'd almost all be ashore at this point, with just a small number of sailors manning the boats.
Jaime takes Highgarden after a siege of undetermined length. Maybe they attacked immediately, maybe it lasted a few weeks, who knows. In reality that castle could hold out a long ass time, so who knows why it fell quick.
So this timeline works out fine to me. Greyworm would have had a big headstart on Euron, but Euron and his fleet are the fastest and most experienced around, while Greyworm and his unsullied are most definitely not sailors (I'm guessing some Iron Islanders were in charge of Greyworm's fleet, but not tons) and the time/distance to go from King's Landing to Casterly Rock would have left ample time for Euron to catch up.
Actually, given that last part - the Unsullied being ashore already can be seen as a bit of a defeat for Euron. If he had caught up before the Unsullied landed, he could have wiped out Dany's entire Unsullied army. Now that the army is fortified, even with low provisions it's a huge danger to Cersei's ambitions.
So at this point, Dany still has the Dothraki in hand, possibly Dornish support (especially if she can't mount a rescue). Highgarden's armies are unaccounted for but we can probably remove them from play. The Unsullied were only marginally bloodied taking Casterly Rock, but lost their ships. The could march towards Riverrun to link up with the armies of the North if Jon and Dany come to an agreement. They probably wouldn't head to Highgarden. The fleets sent to transport the Unsullied and Yara's fleet are totally gone (save for 3 from Yara's fleet), so there's just an unknown number of ships at Dragonstone in hand. Time for the return of Sallador San?