‘We should catch up soon’ expresses goodwill, but it may never become a plan. At the other extreme, offering one fixed time can make an invitation difficult for a busy friend to accept.
Combine a real option with room to adjust: ‘Would you like coffee next weekend? Saturday afternoon is easiest for me, but we can look at another day.’ The invitation has a starting point without pretending your schedule is the only one that matters.
If you are not ready to plan, say that accurately: ‘I’d like to catch up when this project is finished. I don’t know my availability yet.’ Avoid making an appointment in words when you cannot currently honour it.
For a group, choose a simple confirmation process rather than continually adding dates. Someone can suggest two options and a time by which to decide.
Do not interpret one declined invitation as a complete statement about the friendship. Look at the specific answer and, if needed, discuss a recurring pattern directly.
A flexible plan still needs a next step. Before sending an invitation, check whether the recipient can accept, suggest an alternative or decline without having to design the whole arrangement themselves. Warmth and practical clarity can sit comfortably in the same message.