I gather from informed posts on this forum that Barcelona barely has an off-season. That said, I do not know that Parc Guell often totally sells out. I would monitor ticket availability on the park website on a daily basis to see how sales are going. The "Check ticket availability" link on the website isn't working for me (not a new problem--apparently a website bug), but you can go through the purchase process to access the calendar and times available. As I write this I see that Monday is sold out and Tuesday tickets are available at 8:30 AM and from noon until closing. Many of the time slots indicate "last tickets available", so there may be more sold-out Tuesday slots by the end of today (Sunday). For Wednesday there are no tickets available for 10 AM - noon. For Thursday the periods 10 AM - 11 AM are sold out. It appears that mornings are more popular than afternoons, which is fortunate for you. You cannot buy tickets at the park, no matter how many remain unsold.
I visited the Dali Museum in May this year. I bought my ticket online some days in advance. If you'll day-tripping from Barcelona, you will need to coordinate your museum entry time with your train tickets.
You have two rail choices to Figueres: fast trains (AVANT and the pricier but not faster AVE), going to the Figueres Vilafant Station, and regional trains going to the Figueres Station. Figueres Vilafant is about 1 mile from the museum; there's a bus synchronized with the train schedule, but if the train is late, you'll have to walk or take a taxi. Figueres Station is about 0.6 miles from the museum.
The fast trains from Barcelona Sants to Figueres Vilafant take about 55 minutes each way; the regional trains to Figueres take nearly 2 hours each way and sometimes considerably longer. At the moment it appears you will pay 35.80 euros round-trip on one of the fast trains unless you select the 7:05 AM departure. Usually the regional trains are cheaper, but the Renfe website doesn't list them, so I don't know what the fare is.
I was staying in Figueres and bought a first-time-slot ticket for a Fridaiy. The museum was blissfully uncrowded initially, but soon a tour group (or possibly multiple groups) showed up, and the main floor got crowded. It's actually a good-size museum, so I went upstairs for awhile. When I went back down it was a lot less crowded and I was able to enjoy seeing everything. It was a very rainy day, which may have affected non-group traffic.
If you click on an individual date on the museum website's calendar, it will show you how many tickets remain available for each time-slot; it appears that the first time slot is popular but that people don't necessarily tend to buy tickets far in advance. I was surrounded by French-speaking folks at the museum; I wonder whether Friday might be busier than Monday-Thursday.
The co-managed jewelry collection in a nearby building is definitely something worth seeing, and it is free with a ticket to the Dali Theatre-Museum for the same day. I was told that one can see the jewelry museum first, if desired.