Thank you for your time and response!

How can I make Pillsbury box mix dense enough to withstand being in tiers?

Do you freeze the whole cake, take out and torte, then freeze until ready to build? What's your process?

Pillsbury is horrible for stacked or tiered cakes. It's fine for cupcakes but I wouldn't use it for anything else. Stick with Betty Crocker or Duncan Hines I have never had an issue with either one.

The cake doesn't need to be dense to be a tiered cake.  Your support system is what holds the cake up.  You could, theoretically put fruitcake on top of angelfood cake with an adequate support system.

Replace the oil called for with an equal amount of Helman's Mayo.......  you do not taste the mayo, but it makes a WONDERFUL cake....

leah is right........it's NOT the cake that holds up the upper tiers.   Here is a wonderful cake recipe.  Please take time to read through all the comments as there is a wealth of information contained all through the many posts!  https://www.cakecentral.com/recipe/7445/the-original-wasc-cake-recipe

As you can see once you have read the linked recipe you can use Pillsbury mixes with good results.