Using an immersion is a very cheap & easy way to get a feel for hot wfp,.. but it has its downsides. You can't switch back to cold if you're doing any fragile or historical windows,.. and your water will cool during the day. Also hot water running through the pump will shorten it's lifespan.
What I've been using for nearly a year is gas "on demand" heaters. I started with an Arazbao JSD20, which to be honest was just too powerful for a 1 man setup,.. and I've just switched to the eccotemp L5 heater. You have cheap, instant & controllable heat with minimum fuss.