I must say I don’t really understand how that circuit works. It looks like it makes a half wave input which could be why the output is so low.
I have done a lot of work with Nixie tubes and VFD displays that require high voltages and the simplest HV power supplies are usually inductor-based boot converters. Here is a simple one with production files and schematic http://www.smbaker.com/high-voltage-power-supply . You can find other ones in the files section of Neonixe group https://groups.google.com/forum/#!forum/neonixie-l
A few others I had in my bookmarks.
http://www.desmith.net/NMdS/Electronics/NixiePSU.html
https://reibot.org/2011/08/07/intro-to-boost-converter/ .
I use these tiny ones in watches. For $9.99 it is hard to make it cheaper and impossible to make it smaller. It is also transformer based so more efficient than the ones above. https://www.ebay.ca/itm/type-c-5vin-170vout-hv5w-high-voltage-nixie-power-supply-module-nixie-tube-era/141728914580?hash=item20ffb39c94 they make many other models including variable and higher amperage versions. I also have a list of many other small ones on eBay here https://www.ebay.ca/cln/sublime_kustoms/Nixie-Power-Supplies/385065490015
Sorry to not offer much help with the circuit.