What it gets down to is often what we each might like and how much we want to spend to get it.
For a really cheap cover for the stand frame, some like recycled cedar fencing. I think it's kind of the "Texas" thing but some do like it and it is easy to find for free on the local Craigslist.

I do 2x4 frame with Baltic birch on the outside. But with current wood prices I’m not building anything lol

Wood is definitely gone way up.
I rarely ever use 2x4 for my racks. 2 x 3 is cheaper and will work fine. Only rack I have that is 2x4 is simply because the span is 8'.

If you do build the structure out of 2x material what about using pallet wood as a skin?

3/4" plywood with correct joinery will take the weight of a tank with no problem. No need for 2x material at all. Key is correct joinery and the right glue. I use screws sparingly and only when I can't wait for glue-ups to dry in clamps. If I do have to use screws I try to use pocket holes.
If you have doubts about the plywood holding up then use 1x4 to build a skeleton and glue it into the plywood stand.

I just built a stand for my 55 gallon and 40 gallon tanks to sit side by side, I used cinder blocks painted white, and wood boards stained dark brown.
Cost about $80 from home depot (wood is expensive due to covid) it worked really well, and will be a cheaper solution for you since you only need it for a 55

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I'm a big fan of 2x4 frame for structural support, and skin it with 1/4" plywood for cosmetic (& some structural support).

I wouldn't go thicker on the plywood (if already overbuilding with 2x4's), it will just add to the weight and cost.

I covered my 2x4 stand with three pieces of 3/16 in plywood. I glued heavy duty magnets from Home Depot to the ply, and countersunk a magnet on the 2x4 frame so the whole piece of plywood comes right off, and jumps right into place when you put it in position. I trimmed the front panel with 90degree outside corner trim to cover the corners, and painted the whole thing. I had a piece of mahogany trim that I mitered oversize to fit the top of the frame, so there is a 'bumper' that is larger than the tank, like a chair rail, and the tank sits on that.