Very cute quilt but that should be no surprise coming from me.
Before I can offer any suggestions I would need to know:
How large are the cats? The size of them may exceed your battings minimum requirements which leads to my next question.
What is your batting minimum quilting distance? Even if it is as much as 10" I would not recommend going that far apart.
What is the quilt's ultimate use? (wall hanging, a much used lap quilt, a child's quilt that will be well loved and much used?) A much used quilt would benefit from more quilting than a wall hanging.
What I can tell you is that the border fabric is busy and dark so whatever quilting you put in the border won't show much. The same goes for some of the more vibrant cats.
As for thread color that also depends. I see a lot of violets and purples in the fabrics and I always like the look of dark purple thread on black so a purple might be an option for you.
Finally, you will only learn ruler work by trying it and doing it. But the pressure of a deadline will hang over you and impede your progress. You can do ditch work without a ruler but you will slip out of the ditch and that might cause you more angst than attempting to do ruler work. The first time I tried ruler work I did find it unwieldy and cumbersome so just FMQ ditchwork. But I can tell you I was out of the ditch a lot more than I was in. If your thread blends you can get away with it but if use a contrasting thread it will be noticeable. I persevered and tried to experiment with some sort of rule on every quilt and now it is 2nd nature to me. Time consuming but definitely a skill set I would not want to be without. This quilt might be the perfect opportunity to dip your toes into rulerwork by doing it to ditch your sashing. But if you feel the pressure with your deadline is too great, I think I might avoid any ditch work and just quilt right up to the seam line in the background black of your blocks. If your sashing finishes at 1 1/2" or less it should be fine with no quilting but if it is larger than 1 1/2" wide you can do a serpentine in the sashing or circles or L's and E's (cursive tall loop like a lower case L and short loop like cursive lower case E) you could also try wishbones or u-turns. Here is a link to a quilt I did U-turns, some people call them back and forth lines.
https://www.quiltingboard.com/pictures-f5/scrappy-batik-hst-quilt-t293883.html
Another option for this quilt would be a panto or all over E2E design. The fabrics you used and your contrast is good and strong and stands all on its own so this quilt would be well suited to a Panto. If I did a panto on this I would audition threads by pooling them on different places of the quilt top. Sometimes what you think will look great will surprise you and not look so great once pooled on the quilt.
Here is a link to the first quilt I did on my LA after practicing on a yard of fabric. The motifs were all very simple and easy to do so maybe this might give you a few ideas for your sashing and borders. https://www.quiltingboard.com/pictures-f5/my-first-quilt-off-longarm-t77776.html