Isn't the horn already on a relay?
I think maybe they use relays generally with horn circuits to reduce current through the steering column since it earths through there and could damage bearings etc. if there was any arcing.
Yep, it is on its on relay already.
Headlights, driving lights, and the ignition system are typically the only things that may really benefit from additional relays altho I have seen a few "hotwire" power windows trying to get them to move faster on other cars.
The draw of small bulbs as in the dash is usually very small.
The two connectors in the bulkhead connector that carry power to the ammeter and back to the engine compartment are often a problem as they tend to be undersized for the load they can carry and often burn and develop higher resistance after 40 years or so. This can affect the voltage getting to the dash, etc.
Various solutions are available online to resolve that. I simply bypassed them altogether and ran an 8 ga thru a grommet to a terminal block inside and connected everything on the interior to that point.
I like the use of the relay center a lot. That is a really neat solution. I am going to have to back up a notch and find one before I add the relays for the head lights as I already have one for the pump and another for the ignition. Nice to put them altogether in one spot instead of spreading them around.