You can just replace / with *, in pronouns as well: Seine * ihre, Kolleg * innen, jede * r.
[Edit: without spaces inbetween, but otherwise things become italic in here]
This way you are the surest, since everybody is included every time.
I really have no clue where your teacher got this mixing thing from. But all this is work in progress. Societies and languages have to transform and that doesn't need to be a linear process. Imo it's even better if it isn't, because exploration and multiperspectivity aren't very linear by nature and irritation and changes make for good opportunities to think and discuss.
For example sometimes I like saying just one gender, if it makes for good, well placed irritation.
Good movie - do a watch