Orthodontist says I need crown before wearing retainers but teeth has no structure left
So I'm about 2 years into my metal braces treatment. Recently, my orthodontist told me that before we proceed with retainers, I need to get a crown placed on the arrowed tooth(#19).
The problem is that when I was around 12-13 years old, both this tooth and my lower right first molar (#30) underwent root canal treatment. After the RCT, the teeth were filled and crowns were placed.
However, after some time, the crown on the arrowed tooth came off. At that time, the dentist told me that reattaching or replacing the crown would be difficult because the tooth had already been shaved down too much. There's very little natural tooth structure left now - probably only around 0.3 cm above the gumline.
So when my ortho doctor recently asked me to get a crown on that tooth, I visited a dentist few days ago. He told me that even if he placed a regular crown, it probably wouldn't hold properly and wouldn't really be advisable. Instead, he suggested that I go back to the childhood dentist who originally did the RCT and ask about some kind of very thin cover/crown (I'm not exactly sure what he meant).
For a second opinion, we visited another dentist. He said he could place a cap/crown, but when I asked how it would hold considering there's so little tooth left, either he avoided giving a direct answer or I just couldn't fully understand his explanation.
At this point, I'm honestly confused about what the best option is and whether this tooth can realistically support a crown long-term.