What is the best way of setting this up? I read here that you can’t modify the ribbon numbered list button. Is that still the case? I just want to click the numbered list button and have it use my custom sequencing (1., a., i.)
The toolbar button is a poor formatting surrogate for users switching from Word and not in a mood to read the documentation about differences.
As mentioned in the referenced article, you can’t persistently modify the configuration for toolbar buttons (more exactly, it is probably possible but this would require diving into deeply buried configuration files in system directories; I don’t recommend this).
Is your “template” a real Writer template, i.e. a file with .ott extension?
If this is the case, you can either customise one of the built-in list styles, e.g. Numbering 123 as in your screenshot, or create a new list style of your choice.
For your comfort, you can assign a keyboard shortcut to this style so that application is nothing more that hitting the shortcut. But don’t forget to assign another shortcut to No List (Ctrl+Shift+F12 is defined by default but I don’t find it very handy) because user shortcut can’t be easily configured for toggling.
Writer is built on the concept of styles, while M$ Word relies too heavily on direct formatting (beyond paragraph styles). Learn how to work with styles. Writer can be fully customised through templates (.ott files). Templates contain primarily styles but also initial contents.
The main difference between a template and an ordinary document is: as soon as a template is opened, it changes into an untitled document. Thus you can’t overwrite your template, even when you have forgotten to File>Save As instead of File>Save.
PS:when asking here, always mention OS name, LO version and save format.