Wien bridge slowly decaying oscillator
As you've discovered, boosting the ridiculously-low Q of RC circuits up to high values (near oscillation) requires very stable components, and well-controlled gain. It is asking a great deal, and stability is a big problem.

Starting with an LC resonator would be much easier. Starting with a higher Q than RC networks means less gain is needed. But "L" you don't allow.

An alternative resonator might be a piezo element. No "L". Starting Q at resonance is high, requiring little extra gain to extend the decay time , or push into oscillation:

A one-transistor gain element is usually used to make an oscillator. Changing resistor values should allow gain to be reduced to the point where oscillations decay slowly. The only downside is that frequency is determined by piezo physical parameters:







simulate this circuit &ndash; Schematic created using CircuitLab