As per above, if you aren't used to using expanding foam, you risk applying too much.
If you are willing to pay for a foam gun, you will be able to apply the foam (low expansion or not) with vastly greater accuracy.
I recently purchased a cheapie gun for £12 (incl delivery) from Amazon. It isn't brilliant but it has removable nozzles that allow me to squirt foam in to 2mm holes. Naturally, it means that you have to purchase gun grade cans of foam.
Similar
https://www.amazon.co.uk/Bond-Profe...1685104622&s=diy&sr=1-7&ts_id=1938896031&th=1
With a gun you should be able to get a pretty level finish. If you are worried about applying too much, run a line of masking tape on the bricks first. Once the foam has cured you can use a snap off blade to cut away the excess foam and tape, before "caulking" it.
In the past, I have left cans of expanding foam connected to guns for over 6 weeks. They were still usable but I had to scrape the metal nozzle with a small screwdriver to free up the pin. A non gun grade can, I expect to throw away after 2 or three weeks.
If you purchase a gun, also spend £6 on a can of gun cleaner- it is basically acetone, but if you make a mess, it will clean that mess up.
If you want to use white silicone, you may need to undercut the foam (so that it is a couple of mm bellow the surface)- If the silicone seal is thin, you risk seeing the foam colour under it.
BTW, expanding foam is water proof, but UV light makes it become brittle.
Spray the areas to be filled with a mist of water. It will increase both adhesion and curing times.
