Some of the tree leaves look a bit stressed too unless that's their normal wavy around the edges look. No experience with quince trees here but I would guess that you are right about it being a virus-related problem. Hope your tree can get thru it ok.

I saw some interesting research that postulated that apple viruses might be spread through certain fungi.

My tree experienced a severe top trunk break due to the weight of accumulated snow one winter. So that could perhaps have made the tree vulnerable to infection, I'm thinking.

Transmission of apple stem grooving virus (Capillovirus mali) to apple from the soil-borne fungus Fusarium solani, Yan Xia, BMC Plant Biology, volume 25, article number 1226, Sept 2025

I also saw some pictures of infected quince fruit that look like my fruits, in this article:

Reliable RT-PCR detection of Apple stem pitting virus in pome fruits and its association with quince fruit deformation disease , BSPP, v58, issue 2, Apr 2009