In my opinion tankless coils are the worst source for hot water one could possibly have.
Cast iron sectional boiler as a rule do not like heat-up / cool down cycling. Some can take it better than others. Yours has already displayed its dislike of heat up / cool down so I suggest that you do not change the control to what is known as "cold start" but simply dial back the existing control settings.
You do want a separate circulator pump for the indirect water heater and you may need a priority control that will keep the room heating circulator(s) off while the water heater is recovering. You will most likely never know if this priority is working because indirects heat up pretty fast. The priority controller is an electronic controller for the circulating pumps so it can be installed later if you find you need it without opening any piping.