Hi Bridie, as a T!D, not as professional advice or opinion:
Welcome to the world of modern insulins!!
As you may have noticed The Mix insulins are not very flexible in dosage. You change one dose and that affects both the short and long acting doses. That may be one of the reasons you are getting low BSLs.
5 injections per day gave me freedom and flexibility. But I using a different long acting insulin to Tresiba,, one that required 2 injections per day. I could skip a meal and the short-acting insulin that went with it if need be or delay it a bit.
I had more insulin doses with which to fine tune the BSLs.
Yes, it takes more organisation to do shots before each meal, particularly before lunch but once in the routine, the changes were worthwhile.
Not having been on Tresiba I shall leave it up to other members to describe their experience with it.
Best Wishes