Thanks everyone for the comments.
It can be very helpful looking at PCBs designed by other people. I would recommend taking a look at designs by J.B. Langston, Phillip Stevens, Bill Shen, and any others you can find. These 3 have very different looking results and their designs are very impressive. There is always something to learn.
Anyone looking at my boards should note the comments I made earlier about my use of "unnecessarily" wide tracks. I also generally keep component and track spacing above the minimum supported by the PCB manufacturer.
Another thing I usually do is increase the size of via holes above the default in EasyEDA (and above the minimum supported by JLCPCB). I increase the hole size and the pad size. This does limit the density of tracks you can run but most of my designs are not very densely packed so it is not usually a problem. Some designers try hard to reduce the number of via holes down to none (in some cases). I don't make this a priority as it often involves running more tracks between pins. I consider doing that on the solder side to add more risk than additional via holes.
Most of my recent designs have these minimums set as design rules:
Track width 12 MIL
Clearance 12 MIL
Via pad diameter 32 MIL
Via drill diameter 16 MIL
Keeping well above the PCB manufacturer's minimum should avoid manufacturing tolerance issues resulting in fragile and unreliable PCBs. As a kit supplier, I really don't want my designs to carry those risks.
Because I design kits to be assembled and understood by people with a wide range of skill levels I have different priorities to many others here. I generally avoid changing 'obvious' pin associations to simplify the layout, such as maintaining the relationship of address and data lines to RAM chip pin names. I try to keep all the ICs facing the same way to reduce the possibility of confusion. I put a lot of effort into useful labels and information on the silkscreen. I want the components well spread out, where possible, to make assembly easier.
I was very pleased with SC114 as all 5 chips have a different number of pins so you can't put the chips in the wrong place by mistake. A little thing but it removes one more small risk. I'm tempted to redesign the PCB though to bring it up to my latest thinking.
Another thing I like is to completely fill the unused areas with a copper fill (or ground plane). If you do a good job on the power supply routing the ground plane is not generally required for these types of circuits but I like the look of them. A lot of copper also gives them a slightly heavier and more robust feel.
Another thing I avoid is sharp 'concave' angles both on track routes and where they join pads. Many years ago (around 1982 I think) I was involved in PCB manufacturing equipment. A few things that concerned the production guys, such as acid traps, influence my current designs. I assume production equipment and techniques have improved but the same issues may still exist to some extent. Acid traps are where acid from the etching process gets trapped in tight spaces and continues to etch away the copper for longer than it should. Narrow tracks are particularly vulnerable to damage here.
One of the reasons I increase the via hole size is from that long-ago experience. The thought of trying to plate those tiny holes fills me with the suspicion that the plating will be very thin and thus a weak point. I try to avoid via holes in tracks carrying any significant current, such as power lines, but when I do need them I usually use several via holes to increase the copper area. Back in the day the quality control guys took daily PCB samples and measured the force required to rip the plating out of a via hole. I wonder if they still do that.
I while back I wrote a sort of beginner's guide to designing using EasyEDA. It might be a bit dated now as EasyEDA is continually evolving but some may find it useful.
If you have any interest in designing PCBs I highly recommend giving it a try. It can be very satisfying, especially when you get your brand new masterpiece turn up in the post from the PCB manufacturer. Just watch out for PCB design addiction - it is a thing!
I didn't rediscover retrocomputing with the aim of selling kits. I just sort of drifted into it. Now it uses up most of my playtime. My original goals seem to have been put on hold :(