Augmenting an existing hydronic system

I have an under tile floor hydronic heating system powered by a Rinnai combi boiler, which also supplies domestic hot water. The floor heat is circulated by a Grundfos pump with a flow rate of about 10 GPM on the low setting. The fuel source is propane. The system works great and keeps the basement cozy.

I put in some solar panels and essentially have free (not counting the capital investment) electricity, more than I can use. I would like to augment my Rinnai boiler with some gentle electrical heating to reduce propane usage, but still leave the system intact for the heavy lifting when it gets very cold.

I have a couple of options which I am considering. One would be to wrap the line near the boiler with a heating tape such as this:

https://energywarm.com/products/engl-engl-1/

My idea is this warming tape would have a thermostat set a couple of degrees higher than the current thermostat which calls on the Rinnai. That would keep the line fairly warm, but if it can't keep up and the temperature drops, the Rinnai would kick in.

The other option I've read about is to install a small electrical cartridge heater directly into the line. Seems plausible, but I'm a bit nervous about the flow rate, not sure if this is straight forward to handle. The last thing I want to do is mess up my fully functional system.

I welcome your comments.