It sounds like everything is weak and you aren't able to recruit the necessary muscle groups to remain stable.
Learn to get better at different squat variations, including low bar, high bar and front squat. Use one variation for your main work each cycle and get better at it - learn to stay tight. After you finish, take what you learn and apply it to another variation. Keep doing that until your technique feels consistent across all variations. That's the point where you get as much carryover as possible across variations and you're able to recruit as many muscle groups as possible simultaneously: all back muscles, abs, all hip muscles, hamstrings and quads.
Learn to brace your abs. Do weighted lunges and wide stance work to bring up your hip strength. Do wide grip pull-ups or pull-downs, wide grip barbell rows, chin-ups or dumbbell rows, and upright rows or lateral raises to increase your back strength. Learn to keep your chest up, shoulder blades down, abs braced and entire back tight (not just erectors) when doing these. You have to learn how it feels to get all these muscles tight so that you're able to do this while squatting. Work your back with a lot of volume. Get a pump in your back muscles from the pulling work and remember how that feels for when you squat.
Based on what you describe, it will likely take close to a year for a drastic improvement in technique if you focus on the above, which is still a lot better than many years. There is no quick and dirty solution. Stop worrying about "arching" and focus on bracing your abs and getting your back and hip muscles tight throughout the lift.
Remember that a good setup isn't based on how you look when getting into position for a lift, it's based on how tight you can get your entire body.