I should get both but I don’t have them now.

I have a KUAT with a max rating of 80lbs.

I’m planning to strap 2 deflated inflatable paddle boards to the rack. Total about 55lbs. I have tie downs, cam straps and bungee cords.

I’ve successfully driven across the country twice with a stroller on the rack. These are a bit bigger. They are less than my max load, more out of the wind than a bike, and I can strap the heck out of them.

Am I missing anything?

I have seen hitch mounted cargo trays, so what you describe sounds like no issue.

I would check that your tail lights and turn signals are not blocked. Otherwise, what could possibly go wrong?

If a bicycle rack does not violate your state's, or any state that you will travel through, load projection laws then it should not matter what you place on the rack as long as it is properly secured and as Broctoon mentioned does not obscure tail/turn signals you should be fine. However the weight limit of a bike rack is based on using it as a bike rack and that is staggering the weight front to back. So a static weigh that is as far forward as possible is likely okay but a static weight at the end of the bars is probably not.

Around here people have been slapped with fines for having their license plates obstructed by hitch bike racks.

Never

I’ve done it once or twice, with a balloon-tire “Beach Buggy” wagon.
as long as you’re within the load limit of the rack, and you’re not concentrating the load on any single element (other than the tongue/drawbar) you should be fine.

You’re probably going to need a couple more tie-downs than you would for a proper cargo platform, since lashing anchors may be less than optimal.

As far as legality; as long as the tail lights aren’t obscured by the load (except partially by bicycles) you’re good in most US jurisdictions; the rack blocking the license plate is a non-moving at best, provided that the rack is a temporary-use device.

I use my bike rack to transport things like ladders all of the time. I've also traveled with duffel bags strapped on top of the rack, that works quite well. Was on a minivan, drove through a rainstorm without a drop of rain hitting the duffle.

Blocking the license plate can be a problem depending on how bored the cops are. Got stopped by Barney Fife somewhere in Arizona who made me move the rear plate onto the bike itself.

I have a separate tray style rack I use to take stuff too and from the landfill. I ratchet strap two trash cans onto the tray to carry yard waste. Only do that for local trips since it does block the rear lights.