I've been working on the guidance numbers for Q1 and they don't make
sense unless one factors in a huge write-off and a surprise on SBC. I'll post on
this shortly.

I'm not sure what you mean by SBC? I expect you're right about a big write-down. Common tactic for a corporation to throw every write-down, restructuring expense, etc ... into the same quarter so that they can say 'one time charge' and other quarters look much better by comparison.

Here's that forward guidance from the press release.
"
Financial Outlook
For the first quarter of 2023, Upstart expects:
● Revenue of approximately $100 million
o Revenue From Fees of approximately $110 million
o Net Interest Income (Loss) of approximately ($10) million
● Contribution Margin of approximately 55%
● Net Income (Loss) of approximately ($145) million
● Adjusted Net Income (Loss) of approximately ($70) million
"

They have the layoffs and associated charges in Q1, announced Jan 31st, as discussed in this Fool article: fool.com - Upstart is reorganizing again what it means for th

The new reorganization will result in $15 million of charges and a one-time $3 million noncash charge due to forfeited stock awards.

They were already hemorrhaging funds prior to this action: the already existing loss rate (~$60m loss from operations in Q4), with further decline in loan volume, for the reasons you mention. With the reduced loan volume it's not hard to see them getting to the guided figure of a ~$70m loss in adjusted net income.

But, they also guided for a GAAP net income loss of more than twice that ($145m), of which they've only previewed ~$18m in charges for the restructuring, leaving ~$57m in other losses coming in the unadjusted figures.

I wouldn't be at all surprised to see most of that figure coming as a big write-down in the value of their loans on the book, due to realized and projected higher default and interest rates, calculated with a pessimistic eye to the future so that if things don't get as bad as their pessimistic forecast, they can later re-adjust value for a positive earnings surprise in a later quarter.

One thing in the guidance that didn't really register for me before I wrote this post is that they are guiding for a net interest income figure of a $10m loss. This for a corp with ~$530m in cash, ~$1,000m in loans vs ~$1m in borrowings. Shouldn't they have net positive interest, considering they are carrying ~$1,000m in personal and auto loans on their books to go with all that cash?!?