water rushing in sealed system, fit additional air vent?

so, after a recent drain down I've filled the system and bled everything I can. when i switch on the system i get a water rushing/gurgling noise, it goes away once the system has run for a while. sometimes you can hear a noise from the (new) pump that sounds like aerated water going through it.

air doesn't seem to accumulate in the pump though.

If you look at my diagram the red is flow, blue return. i suspect air rises to the high point of the return, then when the pump starts it gets pushed around the system, between boiler and valves.

After the valves, the flow/return dip further down along the garage wall, then through that wall into the floor void. i'm not getting any extra air bled out of any other rads so I assume the pump can't push air downwards and out into the rads?

I guess the answer should be fit an air vent on that part of the return? just want to check if that's sensible before I cut into a pipe.

In case you are wondering why the boiler is there, i believe the boiler used to be on the right above the pressure vessel (red circle) was moved when it was replaced with a condensing one and needed to be near a drain.

AIr separates in the boiler. Put an auto air vent in the flow pipe near it.
One on the return wouldn't hurt.

do you know if auto air vents can work if tilted slightly? the return pipe is directly below the flow so no room for one without tilting it or adding in a bend in the pipe to bring it out.