Bad news OP, your water heater is very likely at the end of its life at 10 years. And expect to spend about double what you think it should cost because I had major sticker shock when mine crapped out in May. My old one was 12 but it was a power vent version and it cost me $2k installed which isn't too far off from the cost of tankless. I knew mine was aging but I had hoped to stretch mine 6 more months so I could budget for it as I already had to replace my fridge a month earlier. The last water heater I replaced was 15 years ago and that was the standard chimney vented one and it was only $600.

My house is 12 so I'm now budgeting to replace the washer & dryer and the furnace next year (I've already been told I'm pushing that now as the builder apparently installed a shitty one). It never ends....