17 years old tankless, fix or replace?
The issues you mentioned are typical problems for a tankless system that hasn't gotten much attention for a few years. Descaling the heat exchanger and replacing a broken mixing valve are fairly simple tasks and can be done in maybe 3 hours by a decent plumber.

Condensing boilers are great and probably much more efficient than your current system. It's worth looking at your usage and figuring out how many years it would take to pay off an upgrade, especially if there are incentives available. But if there are no problems other than the ones you've mentioned, it's unlikely the cost to repair those would justify replacing the boiler on its own. On the other hand, if you're going to own your home for several more years and the efficiency improvement will pay for itself in that time, perhaps you'd prefer to take that leap now and skip the repairs on your current system.