Hi & welcome! Adding copious amounts of perlite to potting soil can give roots room to roam, while reducing weight by volume. I would expect better growth from that than by simply using a smaller volume of soil which would be more likely to inhibit root growth.
Landscape fabric would serve your purpose, available in rolls you cut-to-size needed.
You could use empty water/soda bottles to occupy space w/o adding weight. Empty, upside-down pots can be buried, then easily removed if you want more root-space later.
You could sit pots in the planters, surrounded with a decorative moss, or something like pine cones, if a "filler" is needed. This would allow easy replacement as annuals finish up, and to have a display of plants with varying moisture needs.
Assuming your planter has a hole in the bottom? Excess water that can't drain out could definitely build up excess weight.
Don't forget to check below before you water, or at least holler, "umbrellas up!" LOL!
