High aggregates such as PT and Wet Edge do last a lot longer than plain plasters. When you dump the water to do the plaster, you can change to chlorine.

The energy efficiency is a matter of economics. There is no single answer. The biggest variable is the cost of power followed by the hydraulics of the pool.

Bromine cannot be stabilized like chlorine can be. The Sun burns it off fairly rapidly.

The heater likely just needs the heat exchanger cleaned out. The big question there though is what caused it to get sooted up to begin with? If an inadequate gas supply did it, what will it cost to fix? This could be a gas line size issue, a meter size issue, or a combo of the two. It could also be a choked off air supply issue too. The only way to find out is to have a heater tech check it. BTW, white smoke is usually indicative of a leak in the heat exchanger dripping water on the burner(s) and black/gray smoke is a gas/air mix issue. Age plays a part in deciding if it's a fix it or replace it decision.

If you have a substantial and southern facing roof face, you might want to look at a solar panel solution for warming the pool in the days of cooler temps or running the pool at night to take advantage of the cooler nights cooling the pool in the Summer.

Generally, in areas of drought, large cartridge filters are a good idea as they tend to require the least amount of fill water. A nice alternate is Pentair's Quad DE filter with a DE alternative. I like the 80 and 100 square foot versions. But if you are have a different automation brand, we would certainly need to know that so everything matches. Most manufacturers offer an extended warranty when a pump, filter, and any other new item are installed. Since you are adding a salt cell, this now becomes a smart thing to consider.

While the net is nice resource for finding items at lower costs, you are unlikely to get a real pro to install the gear meaning you would have to and from your questions, I would say you are not qualified. Getting the pricing from a few of the contractors in your area is, IMHO, the proper route.

Scott