15 degrees bank angle in the climb is normal in a trainer like a 150/152, pa28 or 172. 30 degrees whilst climbing just reduces climb performance and stall margin.
The longer you have one wing lowered in a high wing aircraft the more exposure you have to not seeing another aircraft in that blind spot.
High wing aircraft need a good lookout prior to the turn. Performing a sudden eratic manuevre (like trying to turn suddenly onto crosswind or downwind) in front of another aircraft is far more dangerous. In the circuit the most important thing to remember is to be predictable. Following aircraft expect you to do a shallow climbing turn after take-off and they will manage their circuit to follow.
The rectangular circuit is much more appropriate in high density traffic environments as one can easily communicate position and visually sight other aircraft. The flow is predictable so lookout should be easier. An oval circuit requires more attention to just flying the circuit, less time available to look for traffic and more spent on alignment with the runway.
Airline manuals may depict a curved base in their manuals but inevitably most fly a square base to help judge the turn to final or separation with traffic. The curved path is more to keep the aircraft in the circling area with minimum chance of losing visual reference during such approaches.
40 degrees anywhere in the circuit is more an evasive manuevre than a normal one. Low speed with flap and a low experience pilot could easily overbank and get into trouble at low level, especially with disorientating wind conditions. If you tried this in a large aircraft it would yell at you annoyingly until the bank angle was under 35 degrees or so.