By chance was this house formerly divided into units and sublet and then later adapted to single family?
I ask as you have a very odd setup that does not really allow you optimal use of your 200 amp service and is especially inefficient. You appear to have a 200A service feed but it has been broken down into individual 90+45+40+25 (200A) service feeds almost as if once the panel had been used to feed different tenant subpanels plus common loads and then later adapted. Though unlabeled, I presume that the 90A breaker is the service feeding your 125A subpanel and the 40A perhaps an oven to some other appliance or subpanel elsewhere?
If the 125A subpanel is the only other service panel feeding the bulk of your house circuits (save for the few in the service entrance panel) then your housewiring is likely quite old and ideally in need of a refit of you want to add much else, which complicates expanding things as-is because you will really need to audit all your other regular loads to ensure that your added shop loads are not at risk of overloading the 90A feed to your subpanel.
If you had a proper 200A main breaker/service disconnect at your service entrance then you could have simply added another circuit in the combined service entrance and main panel, but because it has no single “main” breaker for your service drop any new circuit would potentially allow the total amperage on your service feed to exceed the 200A rating of your drop, unless the other breakers were collectively reduced by the same total amperage as the new breaker. For example, if you added a 60A breaker in that panel to feed a new subpanel to serve your shop loads then you could potentially draw up to 260A on your drop (90+45+40+25+60=260A), which would violate code since the potential exists to draw more than the rated 200A without an overcurrent protection device tripping.
If you have enough capacity remaining unused to service your shop needs in addition to the existing house loads in that presumably 90A circuit feeding the subpanel then you could remove two circuits to install a double-pole 240V breaker in the existing subpanel to feed another subpanel — but shop loads could easily consume anywhere from 1/3 to 1/2 (25-50A) of that existing 90A feed which means you could need unrealistic levels of spare unused capacity. However, you would need a careful audit of the existing loads to ensure you are not in danger of tripping the subpanel’s main breaker just because, say, the air conditioner cycled in at the same time you also had the water heater cycle on plus clothes drying in the dryer while you are out in your shop running the dust collector and thickness planer and the air compressor cycles on (as a practical real world example). You would need to know, under your typical peak usage conditions (i.e. not everything turned on as that is not typical) what the peak amp load is in each of the two 120V legs to determine how much capacity you really have for your 240V shop loads plus any added 120V circuits you may add.
Your best bet at this time is to break down and actually hire a competent electrician to audit your existing electrical and provide you with the best advice on how to move forward and safely add your new circuits.
It may well be that your most practical option includes upgrading the service entrance panel to one that includes a proper 200A main disconnect breaker and has room for a dozen or so slots so that you can transfer the existing loads in that main panel over to the new panel and still have enough room to add another 60-100A breaker to feed a second “shop” subpanel in your garage/workshop.
