Couple of extra tips and observations.
That Sentry is well placed,but remember how powerful EMP's are.Someone just has to throw one by the red van and it will take out the sentry and the trophy.It's a good place if you already have one up next to fountain or,as if in that case,you are in a hurry,but the ideal place you want it is on top of,or next to,the fountain area.Easier for the trophies to work,better lines of sight for the sentry.
Map control is an ambiguous concept.Think of the map in 4 sections.Your flag-Area before your flag-Area before their flag-Their flag.You want to think of the map as moving forward and back between these 4 areas.Get 2-3 kills,move forward to the next area.Get 2 or 3 down on your team,your remaining members need to rotate back to cover the flag.
Where and how you want to spread your 4 is dependent upon 1.)Your Strat 2.)Their strat.For instance,if you want to run the flag down the boxes/tractor route on standoff,then you want to keep your guys out of back alley,set up cut outs to cover the street area and courtyard area,then rotate as a team back towards your base to cover their guys,which you now know will be rushing from gas station/back alley because you influenced their spawn by blocking their box spawn, forcing a gas station/back alley spawn.
The other tips are useful for your OBJ player,but you should only have one,possibly two depending on the circumstances,running as your OBJ.Ideally you want 2 AR's,a slayer,and an OBJ.OBJ runs,slayer slays,and AR's get cutouts when the flag is run and attempt to hold down map control.
Y'all have the right idea,just a matter of fine-tuning it a little bit.