Correct, but less brands are using this odd 355er rim size, as far as I know are these only:
1. Birdy
2. Strida
3. Iruka

and a few rare children bikes or recumbents, occasionally some BMX bikes or monowheels.

Schwalbe Marathon, size 35-355 (18 x 1,35), was discontinued by Schwalbe already several years ago.
But Schwalbe Marathon Racer in 40-355 (18 x 1,50) and Schwalbe Marathon in 44-355 (18 x 1,65) are still available!

Innova 32-355 is the standard Strida 18" tyre -> https://stridacanada.ca/product/stri...-18-inch-tire/

At the Iruka page
https://www.iruka.tokyo/en/overview/lineup-and-specs
you'll find another 18" tyre in size 18 x 1,5 (most likely is that 40-355 ETRTO), the brand is GMD (Gumonder) and the type G701
https://www.imb2b.com/ebook/catalog/...gmd_2019-2020/
just, I couldn't find a source in your area.

You wrote there is room; maybe you could use tyres up to 1,75 width, such an alternative might be this one:
https://www.einradladen.com/18-x-175...Tire-NLK-White

Good luck

Chris