What do you put each of your tiers on if they will be stacked directly on the next tier if not using the sps system? I've been using heavy cardboard covered with freezer paper, but I worry about the tape. I was wondering if someone has a better idea. I've also had trouble with my buttercream and tiny cracking on the stacked cakes. So frustrating after all that trouble.

You state using heavy cardboard - are you using cake circles? Or some other cardboard?
Many people use foamcore board but I could never cut them nice and straight (cleanly) so never used it. Just a cakeboard is usually good enough.
As for tape used in covering boards w/fzr paper (which is what I usually used) I would cut a fzr paper round the same size as the board and after taping top covering would glue (elmer's glue) the round to the back/bottom.
As for your stacked cakes' icing cracking try adding just a bit more fat (Crisco or butter) to your recipe. Sometimes all it takes to make it creamy-er is as little as a tablespoon more fat. OR some people add a bit of Karo syrup for a similar effect. You want to make the icing creamy-er but not thin it as adding water/milk would. Don't be afraid to play around a bit with your recipe(s). You may not be measuring *EXACT* or it may be more humid/dryer in your area then others. Make notes of what you do - doing one thing at a time - until you are satisfied w/the results and know what works best for you.

I use cake cardboard circles. I do not wrap them in anything.

Indydebi and Kakeladi, thank you for your replies!

I also use cardboard cake circles and I don't cover mine either. I have
never seen a reason to cover them.

It just seems like it would soften without some kind of covering. I cut my own cardboard, so it has to be covered. The cake circles you buy just don't seem sturdy enough.

I definitely trust you and love your icing recipe, by the way!