Unless the thing is installed outside in the snowy passes, the standby losses of better electric tanks are so low it could take you a decade to recoup the cost of installing a tankless at Puget Sound Energy electric rates. Insulating all of the near-tank plumbing within 6-8' of the tank (including the cold feed and the temperature/pressure outflow plumbing) to R4+ with 5/8" wall closed-cell pipe insulation can take the standby loss down even further, and would be also be cost-effective. If you set the tank to 120F, insulate the tank with retrofit tank-blankie + 2" of rigid foam-insulation bottom pad and insulate the near-tank plumbing your standby losses would be on the order of ~$5 month, (assuming 12 cent electricity.) At 120F it won't kill legionella, but legionalla won't grow. Don't park it lower temp than 120F unless you take it all the way below 80F.
Insulating all of the hot water distribution plumbing in the house would make sense if the house were in daily use, and even here it makes more economic & practical sense than a tankless in series. Get the good stuff even if you have to buy it online (or at Graingers)- the box-stores only seem to carry the 3/8" wall pipe insulation.