If you want long-term maintenance projects, then tankless is perfect solution.
BUT <> As with existing installation, sometimes homeowner spends lotsa money flying a pig. We give opinions based on experience, instead of style. Avoid tankless WT error
A tankless will NOT integrate with existing system.
Tankless would have to be separate unit apart from the Lennox installation. To suggest otherwise raises alarm that installer doesn’t understand how various water heating systems work.
Integrating tank-type water heater to existing system doesn’t seem logical. If you can connect ordinary tank-type heater to Lennox tank, why not just by-pass the Lennox tank altogether?
I am wondering: you are thinking about spending $1800-$2500 for properly-sized tankless water heater, plus cost of running new 6″ vent pipe, and running larger gas line, and possibly installing larger gas meter to support tankless combustion, possibly taking huge risk of voiding warranty and messing up big-time by installing tankless yourself without training and experience, plus long-term cost of continual tankless maintenance, and inevitable cost of repairs and parts <> why not replace the whole Lennox with that kind of money?
OR simply find a way to install ordinary tank-type gas-or-electric water heater, and by-pass the Lennox problem.