For me the solution was changing the anode in the water heater. A temporary solution lasting a week or two is to dump a pint of hydrogen peroxide per 20 gallons into the water heater. Link below but do more Googling, Getting the old anode out can be tough though.

https://www.corroprotec.com/blog/sulfur-smell-in-hot-water/

I had the anode replaced, did not help. The anode was ~$100 at a plumbing supply shop. If you do not have enough room over the water heater, one can cut the existing anode for removal, and the replacement will have to be segmented, 2-3 pieces linked together.

I shock my well ~30 days with pool chlorine.

How does one dump a pint of h2o2 into the water heater?

How to put H2O2 in the water heater...

https://www.cleanwaterstore.com/resource/how-to-guides/how-to-eliminate-water-heater-odors/

I paid $29 for a segmented anode but was a real bear getting the old one out. Took a breaker bar with about a 4' cheater pipe to break it free. But now I can easily replace it every few years so was worth the effort.

https://www.amazon.com/gp/product/B007ZI385E/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

Was your replacement anode an aluminum one, think that is the one to use rather than the standard magnesium in cases of hydrogen sulphide smell. Also more frequent use of the hot water might reduce the smell but not easy if only occasional use.

correct - use aluminum. The link above was not aluminum - that heater did not have the sulfur smell problem.

I have thought about an on-demand but know little about them. Something different from a tankless? Bosch makes some small ones to go under a kitchen sink. We don’t have the skill to replace the rod, and it sounds expensive. I need to ask the previous owners how old the water heater is. We may tough it out until this heater dies, then do everything at once.

Thanks, all.

I am referring to tankless water heaters when I used the phrase on-demand.