I reread your original post.
Problem is, sytem works good in the winter but in summer with boiler idiling the domestic hot water needs can suprisze the stoker boiler combination with load.
Possible solutions, that should cost much less than 1000$
1. Since you have an extra zone how about coneting that zone piping with no load on it, then putting a timer in that acts like a closed thermistat for say 10 minutes, now when large hot water demands are anticipated bump the timer to 10 minutes, now the boiler heats itself and the new no load zone, and can handle the DHW load, and in ten minutes if the load didn't happen all returns to normal.
2. Same plumbing as above except put a thermal well in this no load zone, now when hot water demands are anticipated push a button powering a thermal switch that closes keeping the zone valve open until the thermal well comes up to temp, warming the boiler and shutting down at set temperature even if the DHW load never happens.
3. If you are electrically inclined it looks like you could trick the Honywell control on the boiler and get the stoker to come on without the circulator, bringing it up to temp. Say for example a relay that closes the thermistat wires on one set of contacts and opens the circut for the circulation pump on another, this relay being energized with the timer from solution one. Now you would turn the timer when large hot water demands are anticipated, relay moves acting like a thermistat calling for heat, and prevents circulator from coming on, allowing the honywell control to bring the boiler up to temperature, and preventing circulation in heating system for the duration of the timer.
I like solution one for simplicity, and three if you like electrical solutions. All these ideas require anticipating the large domestic hot water load.
Thanks for your time, hope it proves productive.