simple test:

I built 6 trade routes and by turn 100 I've got maybe 10 total routes (mine and theirs ) for maybe 100GP total coming in

the rest of economy brings in about the same at 50% tax rate at that time except for

TOURISM! which has me in the 1-2k range, depending on map size, etc.

Without tourism, trade would be a useful add.  in the presence of tourism, shouldn't really be a consideration

True, trade provides benefits just beyond income.

For a first order game balance what I'd suggest is that tourism revenue be hard capped to be no more than, say 100% of all other income- which would then incentivize adding more trade routes to bump up the tourism revenues.

Additionally, and maybe this is just me, but I think making at least one trade route between civs a prerequisite for any treaties or alliances.

Yeah, I agree with the op. True, it needs time to build up, and certainly early game the player should not get a huge income easily.

But personally I'd like trade to be somewhat better, tourism otoh somewhat weaker in generating money. As things are now it is not a huge problem for me, but it feels always kinda imbalanced.