The situation looks rather simple here and I agree with what others have said so far.
You don't appear to have much room to the right of the current box without interference. The Rachio outdoor enclosure only allows bringing the mains power in from the left underside side of the enclosure. (Note to Rachio, making that internal hard-wire bay reversible to handle retro-fitting existing installations would be nice a nice touch.)
If it were mine I would be extending the electrical conduit further along the wall. Far enough so the distance between the center of the existing irrigation wire conduit (Is there any? Hard to tell from the shots) and the center of a new upwards facing the power conduit outlet is the same distance as the Rachio outdoor enclosure expects. A photo from 5-10 feet away of the whole setup would be helpful to see what you're dealing with.
The other challenge I see is your irrigation wires have that gnarly PVC end cap hanging out there. I suspect it is to keep the irrigation wire bundle off the EMT (Electric Metallic Tube) running along the wall. Is there enough room behind that PVC part and the siding to extend the same style electric conduit behind it without interference? If not, it may make extending the power conduit just a touch more challenging unless you bend it around that PVC cap. You can usually bend the rigid plastic outdoor electrical conduit with a little heat. If it were me I'd come up with a cleaner way of keeping the irrigation wires off of the EMT that allows for extending that other conduit. The way they left it for you now looks really unfinished.
I would open the box where that electrical conduit currently goes back to and fish new leads to the new Rachio enclosure once you have it in place. You don't want to get into adding another junction box there for such a small extension. Any outdoor wire junction you can eliminate is one less that could corrode over time and cause trouble.
Please don't take this the wrong way, but from the post so far you don't sound comfortable working with household electricity. If you aren't 200% sure what you're doing, especially outside, please call an electrician. They could bang this out for you in moments.