Seems like it should be possible to solder an eUICC chip to a SIM-card form factor board and use it with OpenEUICC. [1] Somebody is even selling those pre-soldered on Taobao: https://shop104192953.world.taobao.com/

Later in that thread, somebody also reported that OpenEUICC works with eSIM.me's cards, too. If you have one of these, can you give it a try perhaps?

[1]: https://gitea.angry.im/PeterCxy/OpenEUICC, https://github.com/AndroPlus-org/magisk-module-openeuicc