Youre not the only one! I love those two as well! I think because because Jian Yi and Zheng Xi are so stable and there is really no doubt between them, #lifers. that's why it's sort of comfortable for them to wait. They sort of had an arc close off before the MoTian stuff started getting serious, so it seems planned. Buuuut the next big thing plot wise is likely (sorry if this is a spoiler I am just predicting based on the One Day chapters- and this is how I would write it) it's probably going to be university and He Tian will have to leave. So if the MoTian ship is to sail it should sail quickly.
If I were writing this comic we would spend a lot of time with Jian Yi and Zheng Xi next, and get them officially dating/moving in together at university. While Mo and He Tian do zoom calls every night.