If you're going to design something from scratch you might as well do an ideal design (or at least shoot for an ideal design and then compromise if you have to) where you use one switching IC to make both 12.6VDC and 250-ish off of another winding. The only critical part that you need to find for that is the right SMPS transformer. There are transformers that have lots of windings (like 5-10) so that you can make all sorts of voltages. So you might get lucky and find one that can make ~250VDC from one winding and then parallel all of the others together to make 12.6VDC. You just do what you have to do to make the 12.6 and use that for feedback and then just try to get something vaguely 250. It could be 230, it could be 280, it doesn't matter that much. You can always drop volts across RCs (which is a not necessarily insignificant part of a tube circuit anyway).
But then you do have a bit of an issue with size. If the SMPS is too large, it might go into hiccup mode. Maybe you can work around that with the right IC. But otherwise, you might consider some kind of differential load circuit like a capacitance multiplier that is also part LTP acting as a diverter - loading one side causes the other to decrease current.
Incidentally you can get chokes like Bourns 5900-104-RC that are 100mH, 83mA and 82 ohms. With that and a capacitance multiplier you could achieve 40dB of attenuation at mains frequencies on the HT output. Although for it to mean anything you might have to do something for the heaters as well. Maybe a common mode choke would help. They can handle a lot more current.