A BRT tunnel has a lot of interesting aspects. We had one in Downtown Seattle (five stations, 1.4 miles) for a while until it became dual bus/rail and then rail only.
We added a third lane at each station for passing and breakdowns. This also meant side platforms instead of a center platform (possible with buses using reversed directions). That meant stations needed every bit of the 60-foot street width, vs. rail which can do nicely with maybe 45 feet.
There will be bunching. The ability to pass helps, but some drivers are slow, and a wheelchair can gum things up, and there are generally more moving parts with a million buses. If I recall we didn't allow bike rack use in the tunnel.
If it's a combo tunnel for express routes that don't stop and local routes that do, you might need four lanes at stations. In a tight ROW, you now need 75' or so just for the pit, plus a safety and logistics perimeter around it. (If you've ever worked with a shored hole like that, there are strict load limits around it to prevent cave-in.)
Alternatively you can do a through-tunnel with the stations requiring buses to exit to a different level, side areas, or the surface.
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