Hi - thanks to everyone who has shared their experience (both questions and comments) on these boards. In case this is helpful, here is my timeline. I applied from inside the UK already holding a Tier 2 General visa.
4 Feb: Considered applying for a Tier 1 visa, researched requirements, and started assembling materials (i.e. drafted personal statement, edited CV, created ten exhibits in powerpoint, reached out to recommenders)
11 Feb: Completed online application to switch and paid application fee
15 Feb: Received email notice that application was referred to Tech Nation
19 Feb: Submitted Tech Nation online application as Exceptional Talent with fast track option
11 Mar: Received email notice of endorsement
12 Mar: Completed online application, including payment for application, IHS payment, and super priority premium lounge service
14 Mar: Uploaded all requested documents (i.e. passport, BRP, endorsement letter)
9 Apr (am): Submitted biometrics at lounge, uploaded documents were deemed OK and not rescanned
9 Apr (pm): Received email notice of approval
12 Apr (am): Received new BRP via DX courier
12 Apr (pm): Received paper decision letter via post
I did not use a solicitor, but found some of the terms on the UKVI sites to be confusing. That's where forums like this were very much appreciated. Good luck with your application!