The solution up/over here has been a service upgrade to 400/320 amp. Most of the design professionals on new/reno SFDs with the instant/tankless units.
We had one McMansion, where the owner was asking for three (3) of the 24KW units, and had a POCO request for 600 amp, 3 phase, 4 wire 120/208 volt. There is a lot of POCO line work required, and that equals $$$$
IMHO, 24KW added to a 200 amp, with 123 amp calc load is a potential liability on you, IF you are doing the calcs, instead of a design professional. No intent made to insult your ability, but watch the liability.