Best approach to suboptimal system?

Two-story 2500sq ft. single-family home in Richmond, VA.

Zone 1 (Downstairs): Aquatherm air handler in the crawl space... fed by independent air conditioner and shared hot water heaters.
Zone 2 (Upstairs): Aquatherm air handler in the attic... fed by independent air conditioner and shared hot water heaters.
2x50 Natural gas hot water heaters in garage which

Immediate issue: Water leaking out of threads around one of the (9yr old) hot water heater pressure relief valves. Local pro replaced valve (with tape) - leak continues. They advise unit must be cracked and suggest like-for-like replacement of unit.

Larger issues:

• General insufficient hot water during the winter--lukewarm showers and loooong (e.g. 1hr) heating cycles when both zones happen to call for heat at the same time. Yes water heater temps are set to 140F+.
• Leaking coil in downstairs air handler--$400 recharge every spring. Local HVAC says First Co. coils are notorious and a replacement coil will soon leak.
• No cold water mixing valve--scalding hot water comes out of faucets. (And no obvious way to add one--one hot line leaving the two heaters goes directly into the wall)
• Second water heater is also 9yrs old - may need replacement soon?
• Hate the idea of water running over our heads in the attic...

Contemplating whether I stick with the same suboptimal setup and continue placing parts piecemeal...

Or...

Perhaps a tankless water heater? Possibly a Navien combi-boiler?

Or..

Something else that makes more sense?

Obviously, money is always a constraint... Thanks for any advice!

source https://hvac-talk.com/vbb/showthread.php?2226933-Best-approach-to-suboptimal-system&goto=newpost