An M5, 18-150 and 270EX is a match made in heaven. The Sigma primes give excellent results, especially in low light, but they're not zoom lenses.
Yes, the EL-100 is a great flash too.. a bit more versatile than the 270EX II.. but yes, it's slightly larger. The reason I got the 270EXII is that it was going for a great price (2nd hand, about 1/6th new price), So I really wanted the smallest flash possible (generally I bounce from ceiling if required on my EOS M5 camera). If I do require more 'bouncing' angles, then I can use my Speedlite 430EXIII-RT which while bigger again, is both more powerful and more versatile again.If you like the 270EX, have a look at the EL-100, it's slightly bigger, but has a head that can flash backwards and sideways. Another option is the Godox *350C, that one is much bigger than the EL-100, but half the price.
I added a black piece of tape as a rim about the bottom of my 270EX, that keeps it from throwing harsh light towards the subject when flipped up towards the ceiling. Don't use blue painters tape, the flash is powerful enough to turn it into a blue effect light
However whenever I do 'really serious' flash work, then I bring out my DSLRs (often with multiple flashes on stands, etc). That's why I just wanted something small (and cheap)- and the 270EXII does a great job from initial testing. The built in flash on the M5 is useful in a pinch (where required) - but it's very low powered and obviously can't be used for bounce flash on the ceiling. That's where the 270EX was what I was looking for.
Thanks for the tip about the black piece of tape as a rim at the bottom of the 270EX.. I'll keep that in mind in case If ind my 270EXII produces unwanted harsh light on my subject. Yes, I know one definitely doesn't want to use colour tape with flashes for such a purpose! Cheers!