There are quite a few similar posts on this subject but I'm a little confused about the best way to proceed in my specific project. All my previous regulators have been linear types (eg: 7805, LM317 at best) - nice and simple. For me anyway.

The project I'm working on needs 3 voltage outputs from a Lithium (lipo or li-ion) battery - 3.3V, 5V, 12V.

It has a ARM based system on module with a 3.3V requirement at probably up to 500mA. (that bit is fine - stackexchange has some great posts on that). But this SoM connects to great big greedy sensors which have 12V @ 250mA and 5V at a 2.5A (!). Those rails likely need to be powered off and on independently to shut off the sensors.

Plus the battery needs to be recharged. I can use one or two cells although it might not be up to me. Two cells might be easier given that I can boost to 12V easily and then derive 3.3V and 5V from the 8.4V line.

I'm not sure about the best way to proceed as there's a blizzard of chips out there. I was hoping someone has used a nicely behaved one that combined charging and regulated outputs. For example, Microchips MCP7383 can charge LiIon fine and a chip like the TPS63020 is a buck/boost regulator to give you 3.3V but other chips combine the charger and the regulator which seems a better idea.

If I use a single cell LiIon I get 3.4/3.5V to 4.2V. Is that too tight for 3.3V, especially given the power draw when I turn on the 5V boot circuit. Should I first boost to 5V and use that to get 3.3V? And then use another boost to 12V?