That's something I have to talk with this new customer, I'll try to start with a reduced amount of dedication and then increase it if needed as the project evolves.
Leetcode questions were never brought up in the interviews I've had for the time being. There were the kind of questions I've mentioned in the OP and some code exercises.
Yes, I'm not really thinking about doing freelancing hands on work. The only possibility would be to help them come up with a high level solution, as I had built a very similar product in the past.
Update: I've (politely) declined the offer and told them to 2x the number if they wanted to bring me in. They've asked me if I can work some hours for the rest of the year in order to help the engineering team launch the first iteration of the product and then revisit the proposal in January.
Thanks for all the comments, you're all being very helpful.