How we homeruled this
In one of our campaigns, I played a wizard who used simulacrum, using it from 13th level, through 20th. My wizard pretty much always had a simulacrum.
Simulacrum is a challenging spell, just look at how many posts there are about it here. Many questions relate to renewable resources.
We made a series of houserules. One of them was a general rule that covers all renewable resources, another, possibly also related to your issue, covered rests.
** To your question about treating Bardic Inspiration as different, because the source of renewal was different, we did not. We treated all of a sim's renewable resources as usable until depleted, then nevermore.
Our rules
Two of our rules:
- A simulacrum loses efficacy over time, until only at-will abilities remain. For any renewable resource, the simulacrum can use it until it is gone, but not renew it.
- A simulacrum can take a short rest, but gains no benefit. It can take a long rest, but gains no benefit. If it foregoes rest, it suffers as its original would suffer.
Our intent was to fulfill the intent of Jeremy Crawford's tweet, as discussed in Nobody's answer:
A simulacrum is meant to lose efficacy over time, essentially running out of juice, until only at-will abilities remain.
We came to this conclusion after much discussion, in order to balance the interests at the gaming table. The DM wanted to not break the game, and not bog down the game in tedious discussion. The other players did not want to bog down combat by essentially giving me a second PC, and really did not want to waste session time talking about it. As the wizard I wanted power, raw power to explore high-level magic.
The DM and I came up with this (and other rules), over time, outside of session. We presented them to the other players, outside of session, in chat; and I think some of them may have even read some of the posts.
We occasionally discussed making simulacrum of other characters. I was always up for it, and having these house rules gave us as players some useful guidelines. Furthermore, my wizard, in-character, knew as a result of research that this is how simulacrum works. This was very useful, as it allowed us to discuss it without bogging down gameplay. We never did make sims of other party members, because we always judged the in-game cost (primarily in time; we never seemed to want to do it when there was plenty of time) to be tactically not worth it, so we never had more sims than the single sim of my wizard. Pity.
Applying our rules to bardic inspiration
This is how our rule would be applied to bardic inspiration:
- The sim of a bard will have the bardic inspiration of the original and can use it, but can never renew it.
Note that by this rule, many bardic features would to usable by the sim of a bard ad infinitim; as an example, the bard sim could use Song of Rest, but renewable resources, regardless of the source of renewal, would not be renewable, and could be used but once, then nevermore.
Having the houserule forestalled a lot of discussion. To your point about bardic inspiration being different than other renewable resources, we exactly wanted to avoid picking apart every different thing, sacrificing gameplay for it. In the end, it worked the way it did because that's what we houseruled. It was also very satisfactory to us that in-game the wizard knew how it worked (as sure as we the characters knew anything) by having become knowledgeable about it in game.
Conclusion
Simulacrum is a deeply intriguing spell, but it leaves a lot unspecified. The RAW can be poked at, but our experience was that no amount of RAW examination made the spell playable, but that our houserules did. Any DM might reasonably rule differently from what I provided here, but this is what worked for us.
