Ok first we don’t add values up like that but a cheater, tandem or double stuff is rated for the amperage on the handle so yes that cheater can draw up to 40 amps without tripping on that one leg 20 at each breaker.
What if you put a 60 amp breaker in that panel that and a 40 and 4 20’s , it is possible it would be just fine we add up the loads on the circuits and in some cases may not count one, for example the 60 for an electric heater and a 40 for a AC unit you would only count the 60 the 40 not running the same time as the 60, this is all part of load calculations your 15 amp breaker will rarely have 12 amps on them when they do have that or 15 it is usually for short periods of time. These are additional safety’s built into code and the diversity of the loads makes a difference SO is your box overloaded? Not even close you really could have much larger breakers as my examples tried to show you are probably drawing just about 1/2 the rated current of this sub and your limiting factor is the breaker feeding it if you have a 125 amp breaker feeding this sub it will protect the panel.
You have a circuit that is all 12 gauge wiring and would like to bump the breaker from a 15 to a 20 go ahead , at some point if you start tripping your main you have gone two far but I rarely see that with all small breakers and a few tandems .
As long as your panel is rated for tandems it is fine the buss can handle the load. If they are true cheaters (non current limiting) devices you may overload that buss location but I doubt it with a 15 full size on the other side.