IMO, it's highly unlikely that you will loose your deposit due to Rivian becoming insolvent in the next several years. They are well-capitalized and have pretty full order books for both consumer and commercial vehicles, with the latter under contract.
As to when you'll get the vehicle you pre-order today, it's anyone's guess. To paraphrase H L Menken, "no one ever went broke underestimating Rivian deliveries."