there are two stages to this.
First, from a health point of view, you need the dust contained as much as possible and the dust collection enclosure works best if the dust falls straight down to the floor and is then sucked away.
Second. the router needs a supply of clean air to the top casing (bottom of the router when its in the table) to stop the bearings getting clogged with dust and to keep the motor cool.
So a perfect scenario would be a big box with an open bottom, funnelling into the extraction ducting, and a large hose hose attached to the bottom of the router and fed from a clean air area, preferably pumped in at low speed.
Crazy isnt it? So you normally have to compromise somewhere to get it all into the table enclosure.
Overheating of the router is not normally a problem to a home hobby worker, as its rare to have the router running for more than a few minutes at a time, so the pumped air system can be ignored. If youre a small business and the router often runs for a half hour or more at a time, then overheating could become a problem.
For me, the dust can not be ignored, so the router should be enclosed. There needs to be inlet air as well as extraction vacuum, otherwise there is no airflow to remove the dust. So holes at the furthest away point from the extraction tube works best, There should be extraction at the fence as well.
A short fat hose connected to the router base and fed down through the bottom of the box will supply all the cooling air that the router needs.
And yes, youre definitely over thinking it (lol)