It doesn't matter if the bus wires form a circle or not (otherwise a circle of track layout wouldn't work!).
For practical reasons, have the bus wires go out to where you have the gate/bridge, and then stop. One of them feeds the gate/bridge. When a train runs over the joins it completes the circle (thus proving that a bus circle doesn't matter!).
Joining lots of dropper wires to bus. Three ways, and there will be many many more:
1) screw connectors. There are things such as "earth blocks" which are like a chocolate block connector, except it has many contacts which are all joined in a solid block of metal. There are numerous variants on this, including some on printed circuit boards. Works fine. (*)
2) solder wires onto a bit of random printed circuit board, and bring the bus onto that. Works, can be fine, but can also be messy. Depends how well you solder, how its planned.
3) solder direct to bus wire. There are wire cutters which can bare the middle of a bit of wire, such as these (several makers do them)
C.K Automatic Wire Stripper | Toolstation Having a bare bit of bus wire, the dropper can be soldered to it. This works, but need to keep the red and black wire joins about from each other so they won't short by mistake.
Boosters are a matter of the current draw of the layout, not anything else. You can divide the wires and send them both left and right if that's sensible. A basic PowerCab might be short of power for a layout of the size you are describing, but it depends on the locos, their power consumption, etc..
There is some sense in planning the layout bus so you can insert "district cut outs". Those provide short circuit protection for a section of layout, shutting that down, leaving the rest running. They have a number of other benefits such as limiting total current any one area may be subjected to.
How you divide things are up to you, but it may be certain lines (up main, down main), or may be certain areas (goods yard). Arrange the bus wires so you can insert cut-outs for the potential sections at a later date. The same arrangements mean you can disconnect parts of the layout to isolate and find faults, rather than being faced with "its not working, where is the fault!" problems and no way of reducing the search space.
(* if using any form of screw connector, I recommend you fit "boot lace ferules" on the end of multi-strand wire. Once you've done a few you'll wonder why you ever struggled without them previously! ).