if there is vegetation, you are stuck to image pairs of 6 to 12 days to keep temporal decorrelation as small as possible. But you can combine pairs of different times to create a time-series.
This can be done in the classic sense (as you did): Please check these slides (old, but correct in principal. The last part addresses combination of several image pairs)
or with the more advanced method of persistent scatterer interferometry (StaMPS - Detailed instructions - #2 by ABraun)