Replumb sillcocks to increase outdoor pressure?
I think your best improvement would come from adding the tee before the filter and running the new PEX to the sillcocks.  I would hold off on replacing the sillcocks at the same time as adding the tee because you might not need to depending on the results from the tee. Plus, the 3/4&quot; sillcocks might not fit through your brick walls. Do this in stages.