Hi Andreanne,
for the predefined accrual methods you cannot influence which cost will be considered... at least not out of the box.
With respect to planned cost, a possible approach could be to use accrual method "Cost-to-cost POC" which requires you to maintain the planned cost on sales order item level. There, you could exclude non-labor cost. The downside is that the other accrual method considers plan changes done in the project, this method, however, requires manual interaction to update changes in planned cost in the sales order item.
For the actual cost the approach is more complicated. You would have to use the SAP Cloud Application Studio and create an add-on. There, you have to use the enhancement option SalesAccruralsValuesGet. (Please note: the typo is in the Studio, not in this text). This, however, will take some effort to make it work smoothly. Unfortunately, I do not know anyone who has done this so far, thus the evaluation effort and confirming that this is really feasible would be with you.
Alternatively, you could introduce sales order items to which you account assign the actual cost for expenses and material (by using the "correct" project tasks). For this SO item, you could then choose the accrual method "continue to defer". Thus, the revenue would only get recognized once the project is completed. But, still this would require you to not continue using accrual method 108 as described above.
Best regards
Otfried