We have an oval tub in the master bath, a little larger than a standard tub. When my wife goes to fill it with hot water, sometimes she gets enough, often she doesn't. She likes it hot, and usually runs just hot water into the tub until it is full. Sometimes that's too much hot water, often the hot water runs out before it gets even 1/3 full.

We have a sufficient size hot water heater, about 40 feet from the tub. The pipes from the water heater run through an unfinished basement (unheated) through fiberglass insulation on the ceiling (of the basement).

We also have a re-circulatiing pump which keeps hot water circulation in the lines, so we have instant hot water throughout the house, if that matters.

I am thinking of putting a point-of-use water heater in line with the hot water line that feeds the tub. Not one with a tank, which might be too small, but a small on-demand, inline heater.

Is that a good solution? If so, can you suggest a brand, make or model? And if not, can you make another suggestion?

Thanks for your help. My wife will appreciate it!

First thing I would suggest would be to test the lower element on the water heater .
If the top element goes bad the water will still heat properly although much slower but if the bottom element fails then the water in the top of the heater (where the thermostat is ) will heat up normally but the water at the bottom will only be lukewarm at best.
It will also cause you to run out of hot water fairly quick as the heater is running at half capacity. Ie a 50 galoon heater will only heat 25 gallons