In some vertical wire runs you have nothing to grab, except shift and brake cables, but these move too. Below is a photo with wires held to cables and connectors secured with my cocoons. The connectors are the miniature banana types. The cocoons consist of velcro hoop collars on both connector sides, wrapped around with a velcro hook sheet. You can see there that the banana connectors try to separate and are only prevented from doing so with velcro.
the velcro doesn't look terribly bulky. Not a bad solution.
I normally just place connectors near the headlight, tail light, and dynamo. I can normally keep the wires tied up to the bracket that the item mounts to. If you don't need to make a disconnection at these vertical runs of wire, maybe you can move the connectors near the lights?
There are many things you may want from connectors, including ease of separation, ease of provisional in the field repair if something goes wrong, availability of the particular connectors later in time, etc.
indeed... it's all a matter of trading off the priorities and making compromises between them.
I haven't thought much about field repairs. I solder the wire in my connector pins and use heatshrink to provide strain relief (i.e. keep them from flexing near the area where the solder has wicked up the wire strands). This, along with good support of the wires, has provided very reliable wiring.
I should say that I've worked with electronics and wiring for most of my professional life, and have seen a lot of failures at connectors. Some were my fault, I have to admit. That was where I learned about the damage from overcrimping a wire in a connector pin or terminal lug.
Properly done, wires and connectors can be very reliable.
For hobbyists, it's not easy to learn all of the lessons that I've learned the hard way. Some of the consumer grade connectors tend to promote bad practices and the use of marginal materials. No question that it can frustrating to put something together that works well.
Steve in Peoria