Personally I would metagame the mages.
This isn't an ideal solution on multiple levels. It takes a lot of GM focus for one, which is always at a premium. You are spending more time trying to reign in an overpowered archetype and less time actually running a cool game and moving the spotlight around. While really strong PCs aren't a problem in general, the fact that mages are so strong they demand spotlight time just to justify them not overrunning the setting really IS a problem.
Second, it isn't like... remotely fun for the player. Like I hope this is an obvious insight most people have about human nature, but most people would rather have X, than to be told they have X+3 but then find they actually have X. They feel like they lost something, that they are being constrained, being artificially limited. It stinks to feel under the gun like that at all times. This is part of why 'There basically always is a background count of 2 in the city" is so bad and why good game design assumes the basic task is unmodified.
Thirdly, often many of these solutions hit adepts and mages playing 'honestly' more than the mage abusing the system. Like think about it, if astral security is trying to spot your sustained spells and its going to blow your cover to use them... are you going to try to use subtle illusions to create fun scenarios and use your imagination to solve problems? No. You are going to only use spells when they give you the maximum effect possible and just douse yourself in buffs like your doing an astral ice bucket challenge the second you think the value of casting a spell at all goes above the risk of astral detection. Astral detection also is a bad solution because it is contextualized as 'security to stop magic' but in REALITY it is 'security that stops EVERYONE but ONLY mages can stop it.' Part of why magicrun feels like magicrun is that a mundane has literally no recourse against a spirit they can't see and don't know exists astrally perceiving them sneaking around. Background count literally doesn't DO anything to sustaining mages until it gets so high adepts and regular mages can't cast spells anyway, so if you routinely are tossing rating 8 background counts your really just pushing everyone to play super-sayan charge up mages.
This is why most GMs want to lower mage power, because sometimes it legitimately is a worse solution to have the GM balance the game at the table themselves rather than making things actually balanced in the first place.
3. Mages are a valuable asset Magic is rare in the world of shadowrun. Therefore a powerful mage is a great resource to have. If your mage shows off how powerful he is too often, have the mafia or a corporation find him and make him an offer of employment. Saying no to this offer is....not encouraged. This will encourage more subtle approaches and encourage more teamwork...( I rather not open that door, way to flashy and loud !! Hey Mr. Decker your turn do awesome stuff !! )
This is... honestly a really terrible solution to problems.
Firstly, it doesn't seem to occur in universe. For obvious reasons. If you are a mafioso trying to lean on an independent operator who makes a living being able to murder your enemies with their mind, the last thing you want to do is piss them off and give them a reason to view you as a threat. Runners aren't nobody 13 year old mages to pressure into signing up for a school to office job scholarship program. If you try to force em into the inside, they will use their connections and power to anihilate you for your opposition for free. Worse, those pesky runners talk and are like, lame ass 'egalitarians' or something, and soon you will find if you start trying to kidnap runner mages that runners don't want to deal with you. So what is likely going to happen is that your going to make that offer to the mage, the mage is going to nod, and instantly kill everyone in the room with you for trying this stunt, including you. Like, sure, he may DIE, but the very real odds of the super powered magical assassin who routinely deals with high level syndicate leaders not taking kindly to your bullshit and killing you on principle is real. You don't become a high ranking member of a crime syndicate by being so dumb as to try to threaten your hitmen, because, you know, they KILL PEOPLE LIKE YOU for a living. Don't get it twisted, a mob boss will desperately want a mage in pocket, but one who decides to try to achieve that by burning bridges not just with a powerful asset who already was willing to work with you, but the entire interconnected community they belong to who are in fact more capable of demolishing your organization than the FBI isn't how one gets and retains that position. The way Mafia works is that it attempts to entangle you in obligation; the more you work for them the more they try to make it feel like YOU owe THEM. They try to worm their way in. If a mob boss threatened a street samurai so overtly would you expect the PC to do anything besides casually draw their gun, kill the boss, kill all the grunts in the room, and then take their chances with whatever souped up bodyguard they had? Then don't expect the mage who is more powerful than the boss and is able to summon INVISIBILE BULLETPROOF GHOSTS TO KILL THEM IN THEIR SLEEP to do anything different.
Basically? Bullying someone who can casually alter reality and summon demigods is really dumb and not something a remotely smart character should do in setting.
Secondly, Samurai or faces of PC level skill are equally valuable to mages. Hell, faces probably, from a strict value standpoint to an org, the highest value PC because like... they are good at the things that get you more money and power, rather than just being good at security or whatever. Like if somehow you made any group capable of doing this you violate rule 0: Shadowrunners, aka extremely powerful freelancers who are essentially better than anyone else at what they do who maintain their independence, exist. I mean one of the reason mages GO to the shadows is because it is acceptable for a mage in the shadows to have no attachments, as opposed to working corporate or accademia. Like 'freedom' is literally the first thing the books tell you about when talking about why people who could do anything choose the shadows.
Finally, and most importantly, you are, essentially, threatening to kill a PC because they are too strong and by saying 'refusing this is ill advised' you are indicating the player doesn't have any agency in how this plays out. It should be VERY OBVIOUS why this isn't a solution to the problem at all and is, essentially, bullying someone for picking the mage. It is... supremely mean. Don't do this.