You wouldn't need 300kW chargers, though!

Battery buses have a battery capacity of around 400kWh. Assuming they are stabled for 6 hours overnight, that's only a charging power of 66kW. Suddenly your depot needs a connection with a peak capacity of 3.3MW instead of the proposed 15MW.

This can get significantly better in practice. There's a peak transit demand during commute hours, but that means there are quite a few unused buses in the middle of the day. Those can charge at the depot to take advantage of cheap daytime solar. A lot of bus routes are timed, with a waiting time of 5 to 10 minutes at the turnaround point. Place an overhead charger here, and the charging demand can be distributed across the day. As a bonus, this also reduced the battery capacity needed - and the associated lower weight reduces total energy demand as well.

Sure, bus depots are going to need beefy connections, but that's hardly an insurmountable obstacle. The ongoing rapid rollout shows that it simply isn't a such a big issue in practice.

You're oversimplifying on many angles.

Firstly you have charging losses - and you're assuming that you can charge at the same rate consistently over the cycle of the charge.

Secondly, doing it like that massively reduces operational flexibility. If buses are all late back (bad traffic for example) you would want to charge more aggressively than the 60kW. You can't so you're going to have buses that are low on charge the next day.

Finally, it's all a bit moot. In most areas you do not have 3MW of spare capacity on the LV network to suddenly plug into. You're going to need a new HV connection or dramatic LV grid reinforcement, so you might as well put a decent connection in at that point. The cost is basically the same, most of the cost is in permits and civils.

Your idea to place charging points at turnaround points is also not as feasible as you make out. It's incredibly hard to do that (TfL massively struggles to get planning for a simple toilet block for drivers at turnaround points) and they are not designed in a way to have buses in a certain exact position to charge often. And even if you could if buses are late they cannot skip the turnaround like now as they need to charge. This will cause massive cascading delays down the route for the rest of the day.

Grid connections are the reason rollout is so slow, at least in the UK. There is relatively plentiful funding for it but most depots are now completely maxed out in power availability - any spare capacity has already been used and the LV DNO queue is 10+ years for local reinforcement.