Haven’t seen a tool I’m completely satisfied with yet. So I’m using JOSM with custom filters to show only features on a particular level. (JOSM’s built-in level selector is unfortunately missing important features.)
As a side note, “very old cathedrals” are the kind of project where you’ll likely hit the limits of Simple Indoor Tagging, such as not being able to map walls as areas. You’ll have to decide how to deal with that.
I have had reasonable to good results using JOSM with some of the plug-ins available, especially if you have access to the building you wish to indoor map or can get an old set of floor plans. If you want to take it a step further you can write your own .css style sheet and make the floors different colors so when you stack them with no filter to make stairs and inner walls line up, you can tell what floor you are working without having to constantly go back and forth to the filter.
Is JOSM still considered the best tool to date (21-07-2021) for indoor mapping? I’ve been using the OSMInEdit tool, and I’ve had decently good results. It allows you to pretty easily import pictures of floor plans which are pretty useful.
Does JOSM allow you to import pdf’s or images to be used as backdrops for editing floor plans? Does JOSM have any other features that make editing indoor features easier?
Yes, JOSM allows you to put your own files as backdrops. It’s not very convenient, though. (And I haven’t used this feature myself yet because getting permission to legally use floor plans of buildings isn’t easy.)
OSMInEdit is quite polished, and it’s clear a lot of care has been taken to build an accessible workflow for indoor mapping. So if you’ve tried it and like it, I see no reason why you shouldn’t use it.
For me personally, OSMInEdit’s main downside is that it’s a separate editor. JOSM just ships with so many tools that are useful for both outdoor and indoor editing (efficiently finding+editing multiple features and various tools for creating geometry with precise angles and distances, to name a few). I don’t realistically expect OSMInEdit will manage to offer something equivalent to all that functionality,