I too like socket head, AKA: Allen. The solution is shop for washers in different specs. Here in the US there is what is called "AN" Washers, smaller OD and tighter fit to a fastener.. I also buy whats called "Fender washers" which are overly large and spread the load to a much bigger area. Good for ,,,,,,,,,,,wait for it..Fenders! and other sheet metal that tends to crack or pull thru.
I tend to avoid split washers, but I do have them, Star washers are useful, small serrations help grab, Fasterners are a topic in itself. As well as locking mechanisms. Lock nuts pinch nuts, etc... In aviation, everything is secured, whether a cotter key, lock nut, Or safety wire.
Theres many vendors online, and regionally here in the US is a company called "Fastenal" who deal with industrial grade fasterners of all types.
Myself? I love stainless! So, unless its a full restoration, I tend to replace a lot of stuff with Stainless. Many of the bolts (Coarse or fine thread) has markings on the top and most retailed are grade 2, soft, But you can look for higher strength for certain applications but gets expensive.
So, heres something I started doing for stainless bolts, Since they have hash marks or numbers, not attractive. But take some tool stock tubing in 1/4" 5/16", 3/8" etc etc ID. Tap it and alter it to suit. If you dont have a lathe, you can do it in a drill press, or by hand with a bench grinder, But once setup you can do a fairly large quantity relatively quickly,, But using a grinding stone, you face off the lettering and then round or dome the bolt head leaving the flats on the sides for wrench or socket.
This more closely resembles many factory Bolt heads.
Once shaped, Polish them. Stainless is lovely and when your (Non stock) bike is done, all those beautiful exposed bolts and fasteners really set it off, especially against shiny black paint or powder coating.
I also like, in certain situations, Domed head nuts,, again, in stainless, use a AN washer or a regular sized stainless and you start looking like you might know what you are doing.
I also have a couple bags of plastic-teflon washers. I use these under painted fenders followed by a appropriate stainless washer. This helps on Stainless, chrome and painted fenders, both for cracking and not digging into the paint. ALWAYS make sure your paint is fully cured before tightening down, as it can bulge or wrinkle.