The force 3 sustaining focus allows you keep Increase Reflexes up 24/7 at force 3 with no penalty. However, it is fairly easy for astral characters to notice that you have a spell cast on you. The focus will turn off if you pass though a ward and that requires you to recast the spell. For the most part, you should be fine with it up all the time.
As for casting, I think your GM has your drain incorrect. Mentor bonuses don't help with drain. It looks like you have a fetish for your combat and healing spells. That means you roll 14 dice on drain tests for combat and healing spells, but if you don't have the fetish, you can't cast the spell.
You should likely keep a force 6 spirit of man around. On average, you'll take no drain summoning it. The powers it should have are Innate Spell (Stunbolt) and one other of your choice. A good order is to protect you or use its powers on people. It takes a simple action to command it. It can't go too far from you (I think 500 or so meters) without going on remote service (basically fire and forget ordering).
As for spells, your go-to spell is Stunbolt. You should cast it at force 7 or 9 depending on how lucky you feel. If there's a lot of dudes together, cast stunball. I would cast it at the same force and accept taking drain. Unfortunately, lightning bolt is kindof meh compared to stunbolt, but you can likely cause electronic devices to reboot. Combat Sense isn't that great for this character, but can be handy if you want to buff someone else (like if you're staying behind in the car while someone else goes into a potential ambush).
You'll be overcasting a lot so the damage will be physical. If you're unlucky with the drain roll, the platelet factories will remove a box of drain. They only kick in when you take 2 or more physical damage in a hit.
If you're feeling cocky or need to stay up, use your pain editor. It'll give you +1 Willpower and the ability to ignore stun. However, you may not know how injured you are and your Intuition drops by 1 (lowering your initiative). It may take a free action to turn on (just like cyberware) or some other action. Most attacks against you will do stun damage due to your high armor. Therefore, turn on the pain editor when you want help with drain or you think you'll be shot a lot. Since everyone knows to geek the mage, you'll be a high priority target.
Always first aid and then cast heal. You can't first aid wounds that have been healed. You can first aid drain, but if you do it on yourself you'll suffer a -4 penalty (-2 self -2 for being awakened).
Dark Goddess means that you'll have make a Willpower+Charisma roll (6 dice) and get three hits or escalate a situation. You'll usually fail this roll so be prepared to up the ante a lot
