You may be trying to do too much in one schedule, meaning operational AND project work.
One technique I’ve used was segregation of operational tasks from project tasks by having them in different physical schedules.
For example, we’d configure one schedule for operational work. If for example, 15% of a resources time is to be spent on operational work each week, we’d add resources to operational tasks with an assignment units of 15% (setting up the task as Fixed Units). These operational schedules may have very few tasks, but those tasks may extend for the entire year. What you put in these schedules in terms of tasks is really dependent on what, if any type of reporting/breakdown management may want to see related to the operational work.
As the second part of this technique, we would then have project specific schedules that only contained work required to produce that project’s specific deliverables. To keep the example simple, we’d allocate resources to project tasks at no more that 85%. (100% – 15% operational allocation) Scheduling these tasks becomes much simpler since we’re now isolated only to that project work.
The key is to ensure that resources on the project schedules don’t exceed 100% when combined with their operational allocation.
Different resources could have different operational demands. Joe might need 20% operational allocation while Sue might only need 10%. So manual work is required to make sure that the project task work plus the operational task work doesn’t exceed 100% for all involved resources.
Its a manual coordination process to identify those resource specific operational allocations, but once defined, it greatly simplifies the individual project schedules by allowing them to focus only on project tasks.
As a side note, I used this technique in a 2013 and 2016 Project server environment where resources tracked task progress using the MS Project provided time sheets. Time sheet users didn’t really know/care which task was in which schedule since they saw all current tasks in their weekly time sheet.
Hope that helps