Many heaters like you have utilize a flow restricter to slow the water down so it can get hot enough. That may be a problem in itself. The tempering valve may be shot as well. Depending on design, the coil may have some restriction in it caused by mineral buildup that would also limit the heat transfer as well.
Most any indirect could be installed to provide you with a large quantity of hot water, the brands are pretty much interchangeable - each has it's internal design features and capabilities, but they all generally will work with any boiler. An indirect is often installed on a priority zone to ensure you get all the shower time you want.
New shower valves come in two general types, thermostatically controlled and pressure balanced. Each provides antiscald capabilities. The thermostatically controlled one may contain both technologies, depending on the design. Different pressure on a pressure balanced valve can limit the total throughput or shut it off entirely, if it gets bad enough. It could affect a thermostatically controlled valve in different ways, depending on the brand. In one hotel in London, the pressure varied all over the place, but the Grohe thermostatially controlled valve kept the output temp pretty consistent, but the flow did change. After experiencing that for a week, I bought a Grohe when I remodeled and have been satisfied, but I don't have the same pressure variations you have.