Have to turn up heat to keep water hot to take a shower. Otherwise no hot water for shower. Also happens at other taps. Seems demand for hot water from taps is not enough to keep water hot.
Furnace is a Vaillant. Forced hot water, baseboard, 2 zones heat, tankless hot water. Heat works fine in both zones.
Noticed white, crusty buildup on tempering valve. Tempering valve will not adjust. It is turned all the way up to 170 degrees. Will be replacing next week.
Am I headed in the right direction with the tempering valve?
Any help would be appreciated.

Am I headed in the right direction with the tempering valve?
YES

Thank you for your reply. I will update after tempering valve is replaced.

Tempering valve replaced today. Hot water works fine. 110 degree water temp from taps. Thanks for the help.