I'd need a bit more info to understand exactly what you've cobbled together there, but if I do understand what you've done, your boiler may not live long.
The tank acted like a buffer to help maintain a (small) supply of water and help heat the incoming cold water so you had a decent 'nominal' temperature output. Bypassing it means the outlet temp is entirely dependent on the volume used and the incoming water temp. Try to use more, it's in contact with the heat exchanger a shorter time, and it's cool. First draw after a long time, quite hot. Mixing valves don't react immediately. Your best bet would be to add an indirect WH tank as a priority zone on the boiler for your hot water supply.
Maybe with a little more understanding, something else would come up. What brand and model boiler do you have?