4iiii/Stages LH cranks are a really accessible and flexible option, and they both do GRX810 which is a straight swap for your 600 series. I’ve got them on two bikes because of purchase price and the fact none of the SPD pedals had come out when I got my first. Only thing I wish I’d considered at the time is now that I have 3 bikes with LH crank PMs (including a road bike), I feel like if I ever switched to something that measures both sides on one bike then I’d have to switch on all of them for my training numbers to be useably consistent across all platforms. So it’s worth considering future purchases too.
If you’ve already got the Favero Unos then the absolute lowest cost option may be to put Xpedo SPD pedal bodies on them - search in forum (or pretty much anywhere on the internet) for a tutorial. Again maybe not the best long term option, but could get maybe you going for less than $100.