If you want an AC heater that is variable I can recommend using a small Variac in front of a transformer with the maximum output voltage/current that you need, or the other way around.
You'll have to calculate the power necessary for the Variac and transformer and add some extra power to make sure that it works under full load. If you want this to work with regulation there are Constant Voltage Variacs that use a small motor to control the Variac by measuring the output voltage - with necessary scaling of the voltage in the feedback circuit you can get it to monitor the output voltage from the transformer too, not just the output voltage of the Variac.
There are also low-voltage Variacs which require a transformer in front of it to work, like the ones from Thalheimer Transformatorenwerke sold by Conrad, the KSS103 (42V@2.5A) and KSS105 (60V@5A) are just fine to use after a suitable transformer. Sizes and data here:
https://ttw.world/index.php/ess-sing...tion-mode.html click the arrow on the bottom line (Input voltage 42 V; 60 V and 230 V; output 0 up to 42 V/ 60 V and 250 V) to see data, click the Variac number and then the arrow to see the sizes (70*55mm and 94.5*58mm).
It is not cheap nor is it small but it works very well whichever of these solutions you go for.
I have an isolated Variac for school use which works with the full mains voltage which I have used in conjunction with a transformer to get low voltages and high currents and it works like a charm.
/Martin