Running 3.3V MCU from LIR2032 Lithium-ion button cell
Under most circumstances, efficiency might imply a switchmode regulator, but the lowest quiescent current available is about 3µA or so, as with this regulator.

As that is quite a deal more than your circuit uses for most of its life, an ultralow Iq linear LDO regulator may be better suited to this application as current from the battery is the key here for long life.

The lowest Iq regulator I have found (I have some products in an energy harvesting application) is the TPS783, with a headline 500nA quiescent current.

There are other options, such as the NCP4681 with 1µA quiescent current.

I am sure there are others out there, but 500nA for a regulator is extremely impressive.

If you set V(reg) to 3V (the nominal minimum for most 3.3V devices), then you should have a battery useable to &lt;3.2V

As you intend to use some solar energy, an Energy harvesting solution might be the way to go.