TL;DR: the auto-attack damage and stats bonus you get from Relic+1 compared to Relic is much more significant than it seems, since, remember, almost everything you can do scales off your weapon in the end.

There's also replacement logistics to account for, yes. The only way to get an equivalent weapon to Relic+1 is BC Turn 5, which is currently offline and hasn't been beaten yet. It is therefore a very safe investment.

On the other hand, investing in your AF2 chest, gloves, pants, accessories, etc, is a potential risk, because you COULD acquire identical items in BC Turn 1-4.

In other words, spend your Myth tomes on things you can only replace in Turn 5 first, it's the safest investment.

Also, Relic+1 glows.