State leadership would still have control of their state. As you point out powers not given to the federal government belong to the states, but as it stands now if the public wants something they need the blessing of the states to get it which is entirely too much power. It allows tiny states to kill attempts by larger states to enact policies because even though the larger state would have more than enough votes to get something through Congress, the Senate treats all states as the same, despite some states representing a tiny fraction of the population. It literally allows a tiny group of elites to override the will of the people.

City/county leadership is focused on how best to keep their city/county running for maximum benefit of their population. States have the same focus over all the cities and counties therein.

This is not even remotely true. They focus on what will first not cause them to lose elections, and second on what will most benefit themselves. Thanks to a finely honed playbook of dirty tricks there's very little these days that will actually cost politicians elections leaving them free to maximize their personal profit.

It is also relatively stupid to put everything to a mass democratic vote, especially for things that should be decided by experts.

That might hold water if experts were deciding those things, but they aren't. Instead those decisions are being made by the highest bidders. Eliminating the Senate wouldn't entirely fix that problem but it would help at least a little. A congressional representative is much more concerned with the needs of his constituents than a senator is and its much harder and more expensive to bribe a majority of Congress than it is to buy a majority of the Senate.

The US is no longer a loose republic, states are tightly bound to the federal government. The US was forced into that position because a bunch of assholes threw a hissy fit when other people said they didn't think they should be allowed to own slaves. They hadn't even been told they couldn't, just that it looked like things were headed that way. So because a bunch of the states demonstrated they couldn't behave like decent human beings and instead acted like bigots (and still seem to be acting like bigots to this day) the federal government had to step in. The US isn't quite a democracy, but it's closer to being one than it is to being a republic, and it's about time people realized that.

The Senate makes no sense because it's a relic of the pre-civil war government that serves no useful purpose anymore. The current state of dysfunction in the US is ample demonstration of that. The fact that we have a political party whose entire policy for a decade now has amounted to stripping rights from various groups of people and blocking or reversing literally every piece of legislation supported by the other party (even to the extent of blocking their own legislation if the other party supports it), and that they've been successful at that despite a significant majority of the public opposing them is even more evidence of why the Senate has to go.