The Panaracer Pasela 650Bx42 rear tire on my Lejeune has become lumpy and no longer spins without rubbing the chainstays, so I am shopping for a replacement. I am over Pasela's at this point: I have used them in a variety of sizes for years, but I have had bad luck with the 650Bx42's. A non-lumpy 42mm tire would work, any recommendations? I guess I could go to a slightly smaller size if necessary. Does anybody make a smooth-tread 40mm tire these days?

Panaracer Gravelking comes in 650Bx38 and 42
I run Gravelking slicks and they are great tires.

You're probably not looking for the lightest fastest tire out there if it means less durable. But if you are, that's the Grand Bois Écureuil ("squirrel"). (Use a browser that translates Japanese to English automatically, like Chrome or Brave.)

38 mm nominal, I forget how accurate that is IRL. Scary-light, super supple. Combine with TPU inner tubes and you'll have lower RR than most pro racers (wild guess, don't hold me to that!)
At my weight, I'd only use them for a big event, like trying to keep up with my ex-racer buddies on an organized ride without enough training. With brand new ones fresh outta the bag. But my wife gets plenty of miles out of them — daily drivers for her.

Blue Lug says "Perfect for commuter bikes and touring bikes." I disagree, terrible choice unless you're a very light rider and/or have a high tolerance for flats. The tread and the casing are both very thin.

BTW buying from GB in Japan is not difficult and the price with shipping is surprisingly affordable, loads cheaper than a Rene Herse purchased in the US. Shipping was surprisingly fast too. Just put it in your cart and check out, then wait for them to email you with the shipping cost. They'll send you a very polite email promptly (maybe next business day, no later) Oh right, maybe not as cheap now with tarriffs? Dunno. But all tires are imported, so tarriffs will come for them all eventually. Maybe buy stuff that's already landed here until they're gone. <sigh>