We had a California king water bed back in the 90s. Our downsizing was in stages as follows:

When the water mattress leaked, replaced it with a regular mattress. Because if the water bed structure, no box springs was needed.

When we moved and the wooden frame was heavy, we removed the large headboard/shelving section.

When we moved and the frame was still heavy, we cut down the sides by half, so it no longer had the pleather covered padded edges that used to hold the water mattress in place.

When we moved and there were too many pieces to carry and reassemble, we simplified the structure to remove about a third of the bottom structure. It wasn't needed for the support of a regular mattress.

When we moved and were tired of carrying heavy wooden pieces, we completely replaced the structure with 2 folding metal interlocking pieces that are so light I can lift one with one hand.

Now we have a 20 year old California king mattress on these two minimalist support pieces.

It helped that along the way my husband had wood working skills to make all the iterations of this bed.

We are thinking of replacing the mattress. It's old and getting lumpy. And I would love to go simpler. Options are interesting. I prefer minimal light transportable ... But I don't want to commit to getting down on the floor as we are both in our 50s. We can do it now ... But what if we can't in a few years, or have back pain, or surgery...

I'm really interested to follow this thread for everyone's ideas.