adobe illustrator - What is the recommended way to design a standee for print, in terms of dimensions?

I need to design a standee 3x4 feet (36x48 inches at 300 DPI). The main element would be a 10-15 MB photo (4000x5000 pixel dimensions approx.) inside it and a text heading and 1-2 design elements.

The problem: When I create a document in either Photoshop or Illustrator for print (36x48 inches, 300 DPI), it is extremely slow in Photoshop. Illustrator still manages it but I also need a little Photoshop work.

The main problem occurs in Photoshop while creating any shape or writing text. It's very slow.

The confusion: Is it a right way to design at actual dimensions (say 36x48 inches instead of 12x16 inches inches)? Or most designers design at small size and later scale it up? (Unsolved)

Confusion 2: If I design at small size in Illustrator, but use the actual image, would scaling it up (until the image is not scaled up more than its original dimensions) for print give the same image quality, OR has it lost its quality already when scaled down (since text would create no problem, easily scalable)? (Unsolved)

Confusion 3: I need to save the work in PDF file. Now, if you design at small scale (say 12x16 inches instead of 36x48 inches) in Illustrator, is it possible to scale it up while saving as PDF (I couldn't find such option)? (Unsolved)

Confusion 4: It's possible to scale up the design in Illustrator (not sure about PDF). But there's no such option in Photoshop. Does it mean you should always design at actual dimensions in Photoshop? (Unsolved)

Confusion 5: If i place the assets in my laptop's SSD (currently in Hard Disk), would it increase design speed? (Unsolved)

My laptop specs: i7 8th Gen processor, 8GB RAM, NVIDIA 940MX GPU.

PS: I know there are too much confusions and queries, but I honestly couldn't find answers to these specific things. I believe all of them are somehow related. It might be very useful for future.

EDIT: I read 3-4 top answers on the question that is referred here, it helped me, but it solved another problem that is not directly asked in this question. My confusions are different, and still are not answered. I've edited the question detail wherever it was necessary to explain the confusions better. Please remove duplicate tag.