I haven't really extrapolated or mapped out potential impact, particularly assessing game breaking ramifications, but I'm imagining there may be some "been there, done that, still doing, never doing that again" perspectives, and I haven't found anything in the forum yet speaking to it.

It seems to me there are a lot of spells that would function well with the ritual tag (examples: necromancy spells where unlife is created but also the spells related to resurrections, another area would be conjuration/summoning magic, I have yet to delve deep into Artificers but they're work seems largely invested in crafting which seems to be a sort of ritual, divination clearly lends itself to ritual use).  I don't believe the RAW neglected such magic but rather presume play test may have indicated some sort of game breaker potential if allowed.

That said, has anyone expanded the ritual magic list in game and want to share how it played out?  

In terms of DDB Mechanics, I don't know how necessary it would be to assert this in the character sheets, but I presume I'd have to rewrite the spells I want to "ritualize" as home-brews and tag them as rituals.   

Related question, if not for ritual necromancy, how else are literal "armies" of undead created?  Under the spellslot system, cocktail math tells me a 20th level wizard with spellslots alone could ride herd over 101 skeletons or zombies.  That's something, but not exactly epic. 

I've started toying with this in my game, and I do see the potential for "Sorceror's Apprentice" level shenanigans, but I want to enter the topic and see if anyone has more substantial experience beyond my speculation and initial foray, or point me to any sage advice or publication which may justify a "just no" approach to enhancing the magic system through expanding ritual magic.

For D&D Beyond mechanics you're right, you'll need to create your own modified spells with the Ritual tag added, as unfortunately there's no way to add a spell as a ritual, or even to customise it.

As for making spells rituals; I think the question you want to ask is whether a spell is something that should be physically taxing to the player. A lot of current ritual spells are things that don't seem terribly demanding, you're just being given the choice of taking an extra 10 minutes or doing them instantly at an energy cost.

I think if your goal is to have a player commanding an entire army of the undead then it's better to just leave the existing spells as-is but maybe homebrew some extra rules around that, for example, if they can construct some kind of power source to keep their undead animated, then you could say that if a raised creature is in the player's service for a certain amount of time, then the spell is no longer necessary as the "Crystal of Unlife" takes over; this way you've still got the rules as written for a short time in which enemies could interfere.

It also depends what your goals are as DM in such a scenario. One other option is to have each "creature" raised actually be a hoard, where each point of HP is actually one zombie, they just act as a group that can be killed and raised again in larger numbers for the "epic" feel without necessarily being unbalanced.