It depends on the scale of the project, however, one approach you can try in the following areas would be:
Ensure that you've identified and agreed with relevant stakeholders what the mandatory requirements are. If you didn't complete this your project simply wouldn't be functional and/or is unlikely to meet the business case.
This gives you a skeleton that you can start to build upon. This can help influencers feel confident and also give you the scope to prioritise the other features.
It's important that you've thought out (and tested) the potential user flows and actions at each step in the journey of your project. You can then progress detailed UI/UX "just in time" (i.e. before the start of a sprint or a time frame your developers are comfortable with) whilst still making sure the overall experience is on the right track.
It is best if you can embed your UX/UI person(s) into your team. The devs should see the designs early at a wireframe stage to help feedback and understand the technical implications of what is being proposed.
Estimate and commit
Estimations can be tricky. You need to understand at a good level the implication of a story from both a dev and test point of view, alongside this, if your team hasn't been together for long, story points are subjective and therefore it will take you a few sprints to translate them into anything plannable.
I try to look at it at an epic level, running the devs through the proposed journeys at a high level and then get them to guestimate. It could be in sprints or t-shirt sizes (e.g. it's similar to project X, which was a large, so it will probably be Y months).
You'll get a sense from there if your expected timeline is unreasonable or at risk. It's important to manage stakeholder expectations, one easy way of doing this is explaining that you can't provide detailed estimations at this stage, so it's likely to be + / - XX% tolerance.
When you start to do regular refinement and planning sessions, you'll get a better sense of how much time it will take you to get through your project backlog.
Have a demo session at the end of every sprint or at the completion of an epic with your relevant stakeholders. Take them through what the team has accomplished and also share the challenges you've faced. Doing this frequently prevent big surprises later on down the road.