Seems like a decent itinerary... trying to pack Chiapas into there is where you'd run into problems.

One thing you might do is fly directly to Oaxaca on your first day instead of just overnighting in CDMX. That would give you four full nights in Oaxaca before you head off, and save you a bus trip. Note that there aren't really any buses going to the coast, it's either a minivan (around 5 hours) or flight (under an hour).

Supratours are not the only bus company operating in the desert regions, I have travelled on "Car Publique".....public coach, these are local buses but won't always go all the way in one journey, you may have to break up the journey and transfer to other buses or Grand taxis and this will diminish your time scale waiting for connections, for more precise information go to the Gare Routiere Bab Doukkala in Marrakesh ( main city bus station) and the same in Ouarzazate or Errachidia.

Another observation, one day is insufficient to explore Fes el Bali and Fes el Jdid medieval Medinas, the Mellah (Jewish Quarter) and the beautiful Jnan Sbil gardens, you need at least four days.

I was there last week. We got a bus past the hostel. It looked like it's been closed for while. There didn't seem to be any accommodation within the park any longer. We stayed in Wulingyuan at a hostel.

I also recommend visiting "The Grand Canyon" to walk on the glass bridge as it's good fun but a little unnerving. It's a 1/2 hour bus ride from Wulingyuan.

I'm in Shanghai on week 7 out of 10 week trip to Southern China. The LP has been good but some info needs updating.

Far too much of Australia is too hot for too much of year ... like living in the Mojave Desert ... which is why we gravitate to the coast.

There are only small parts of the country (effectively alpine regions, Tasmania, and within a few hundred km of here in Melbourne), with anything even approximately approaching four seasons.

Have visited Tavira a couple of times now - two nights on first visit, seven nights on second visit.

Used buses or trains to explore the area, but also took a couple of river trips.

First was from Tavira to Ilha De Tavira, second was from Villa Real across the river into Ayamonte in Spain.

Had a good wander around and lunch in Ayamonte before getting the ferry back to Villa Real.

Ferry left Ayamonte at 15.00 and arrived in Villa Real at 14.20 - time travel!!!

Was in Sevilla for four nights last week - also visited for five nights last year.

GF took a three hour cycle tour organised via Pancho Tours and thoroughly enjoyed it.

Relaxed pace, plenty to see and €25 per person - I wandered around city centre with a camera whilst she was on the ride as a) we’re not joined at the hip and b) I can walk, but no longer ride thanks to a blue light job a few years ago.

They also do a Tapas Tour ride too - this was apparently €75 for two people and included a choice of three tapas per peson...

Real Alcazar a good place to visit, as is the Metropol Parasol.

GF visited the Catedral in Seville last time around and we both spent time wandering around Plaza De Espana and the adjoining park last week - Plaza is quite impressive.

Loads of places to eat in Seville. Five tapas and two drinks each came to a total of @ €32 last week at Pelayo Bar.

Four tapas and two drinks each at Carmela Bar was just under €30 last week, again for the two of us.

We found both places whilst wandering around last year and have now stopped off at both a few more times....

Other tips?

Good walking shoes as Villa Real, Faro, Ayamonte, Tavira and Seville are best explored on foot.

Eating out in Seville was usually done much later than we do at home in the UK.

Although places were busy last week in Malaga, Ronda, Cadiz, Jerez and Seville we didn’t have too long to wait to get a table.

Some places were basic bars, some were ‘Artisan’ cafes, some were tapas bars and there were a trio of Italian restaurants too. Food was good and cheap in the basic bars and good, but still reasonably pocket friendly at the others.

You don’t say when you’re going.

Daytime temperatures over the last two weeks have ranged from 25C to 37C, so SPF 30 to SPF 50 sunscreens came into play, as did carrying a bottle of water each.

We had spotted the potential for thunderstorms on the advance forecasts and took good rain jackets with us.

Got slightly caught in rain in Cadiz, but missed the floods in Ronda by a few hours - our bus out of the town left at 09.30 and the rain arrived later in the day.

Have now made six visits to Portugal and Spain in recent year, but have yet to hire a car as we use buses or trains on longer journeys and whatever local transport is available to explore villages, towns or cities.

Hope that this helps!

Florianopolis is at least 16 hours by road.
Buzios is 2 to 3.5 hours by bus, depending on which departure you choose.
A good day trip might be Petropolis, not such a long ride (1-2 hours)from the bus station.
If you do indeed get stuck in Barra de Tijuca, you can explore the nearby Reserva de Marapendi, an interesting wetland area
The gorgeous cove at Joatinga beach is roughly at the Rio end of Barra de Tijuca beach. (At lower tide) Get a taxi/Uber into the condo complex and walk down. Google photos online.

The two little piles of sand in the south-east of Morocco aren't as impressive as those in Arizona,

Admittedly, I've been to Arizona and Sonora desert, it was amazing there, especially then late Monument Valley. But I haven't seen any high sand dunes there, if I missed them or?

Those "two little piles of sand" are fantastic and amazing desert, Erg Chebbi with high sand dunes up to 150m playing with all possible shades of red and pink color but also white and gray (depending of the sky). Big desert around there until to Erg Chigaga where sand dunes are not so high but imposing in any case.
With your 18 days you would have time to go and explore, and compare it with Arizona. It is any time interesting to see all different landscape even if it is of the same type. I highly recommend it to you.
On the way you have many other interesting places to explore also.