'more Reliable & Efficient'

By adding the micro you are adding much complexity to the device - what was a handful of transistors,resistors & relays all are very reliable and easy to understand devices when correctly designed with a micro which is inherently more complex and adding a layer of software which may have defects along with the possibility of it resetting when there is a glitch.

Triacs require a lot of protection to survive high voltage spikes and overloads - relays are much more tolerant. If you want reliability -choose a relay.

Efficiency - to make the existing circuit more efficient you would most likely have to use a different technique. Then we get back to reliability.
Transformaers are fairly efficient - you'll have to try hard to beat it!

Having said the above, if you need a cheap micro with a/d ,have a look at Atmel's AVR (at90s4434 or 90s8535)
(yes I know this is an 8051 site!! but where are the small 8051's with a/d like the Pics,AVRs,HC05s? yes there is Cygnal but that is a larger capcity device along with the pricetag). There is a choice of free and/or cheap assemblers & 'C' compilers. See AVRFreaks (similar site to this for AVRs). I use a variety of micros, each has their good and bad features - depends on the application. The more you try the more you'll learn.

For an exercise, replacing the old circuitry with a micro would make a good beginner project where you'll learn a lot about the realities of electronics. Give it a go, you'll be better skilled for it.