Even if you're effectively 'just' a tech lead of a small team, like I am, it's still a very different job than being a tech lead of a small team in a larger company. You are at the top level of the small group, so you end up being involved in a lot more non-tech things than you think you would be.
Your peers are not other engineers and engineering managers like you would be in most situations, but marketers, product people, designers, PMs and so on. Who is the founder and ultimate authority (CEO) and if you are a co-founder or not also really changes things. A founder CTO is also very different from a non-founder CTO. Co-founder conflicts is a huge thing to manage, (see https://flocrivello.com/co-founding-considered-harmful/ ) because at a small size even as a non founder, you're pretty close to it. You should decide upfront if you're going to approach the job as another effective founder or employee and communicate that to founder at the start as part of a conversation.
I made the transition from staff eng to eng manager to CTO at a small startup, and each one is still a very different job even though the skill sets transfer greatly. I code way more at the startup than I did as an eng manager too. Several things I would suggest:
1. You're an exec, even if your company is tiny, and how it works is different than being a manager. Concepts like the 'first team' https://www.michaelvizdos.com/resources/first-team and 'getting on the balcony' matter way more than when you're in a purely product & eng organization: https://www.bettermanager.co/post/move-away-from-the-dance-f...
2. Don't be shy about getting exec coaching, and having the company pay for it. We did that and it was very helpful.
3. Read Radical Candor (updated edition!!!), but also read it with a bit of a grain of salt, realizing the title and advice was chosen because the writer wanted to balance their general high agreeableness with a counterweight that goes too far for the typical person IMO, which the writer writes about that exact dynamic in the second edition.
2a. Learning to be properly direct is super essential, especially at small sizes where its more make or break based on how emotionally deluded you are. The people you are working with need be able to handle you being real with them too. If they cannot, move on, it's that important.
4. Expand your skill set, do everything technical. If you're a backend engineering manager, expand your stack to all parts of the business. That means the back end, the front end, data, analytics, observability, devops, AI models, performance tracing, etc. I started as a mobile eng with previous backend experience, and I wish I expanded sooner so the company had those things on lock sooner, especially in the data analytics side. Engineers hate analytics, embrace it fast to counteract that tendency.
5. Gergely, Will Larson, etc are good resources, but also note they come from what I call the "Uber strain", and thus structure a lot of their experience and advice based on formative years working at Uber. I also worked at Uber when they were there and I recognize a bunch of Uber-isms in their writing.
A good chunk of what they talk about is how to be successful at a place like Uber. Something I've realized later with them is that other companies can be pretty different, and what they espouse sometimes might not match. But on the other hand, Uber's work culture came from previous strains of silicon valley tech culture, especially google's due to the top engineers being from google and setting up their promo system to match googles (vs lets say apple) and the knock on effects of that. Something about that place made people get really good at understanding what is needed to succeed in tech leadership and write about it, I guess it's kind of a paypal mafia effect.