Really helpful input, everyone - thank you! That is why I join forums like these. I didn't even know these companies exist!
I have ordered a strap from B&R (navy calf leather), a strap from Delugs (royal blue leather), 4 straps from Barton Watch Bands (1 black alligator leather, 1 blue leather with white stitching, 1 navy elite silicone and 1 bright grey elite silicone). I've got my eye on a Hirsch and also a NATO from Crown and Buckle.
I've also done something by trial (that could turn out to be trial and error) and purchased a 19mm Tropic rubber strap from YEMA. I have no idea if it will fit an Aqua Terra but it will only be a 60 euro mistake if it doesn't - not the end of the world.
I have definitely been proven wrong in regards to 19mm leather but my complaints about 19mm rubber still ring true unfortunately.
Being that this is going to be my one-watch collection for the foreseeable, I am really pleased at how versatile and subdued yet playful the blue dial of the Aqua Terra is. I don't think I could've done better with a £5k budget for an every situation daily piece. It is also why I am comfortable getting a bunch of these 19mm accessories to be able to dress it up and down for whatever I am doing or what season it is.