We've been advised, that we shouldn't create a project for everything, if a job is too small to perhaps warrant such a configuration.

I confirm 😉

So if an Activity is maybe 1-2 days, it could be an activity of a project called "Mini projects".

Yes, good choice 🙂

The issue I'm seeing here, is if I plan 1 of the "Activities" in this project... there is no way to finalise this? The next time I add a small activity to be planned, it may well affect the other activity i've planned/agreed with the client?

If your first activity has higher priority (lower value of "priority" or just samller WBS), it will be planned before the second one. So the second one should not interact with first one.
You can also add a dependency to force the second one to start after the first one.

Ok, but what if:

Activity1 is a fixed date (can't move) because for example we've arranged it with 30 people, a big meeting and simply something that CANNOT slip in terms of delivery. It's a specified time/place possibly with people around the world.

Activity2 comes in and needs to be planned "ASAP" but uses the same resources

There is no risk Activity2 might accidentally reschedule Activity1?

Activity1 is a fixed date (can't move) because for example we've arranged it with 30 people, a big meeting and simply something that CANNOT slip in terms of delivery. It's a specified time/place possibly with people around the world.

Just record it as a Meeting :woohoo:

Activity2 comes in and needs to be planned "ASAP" but uses the same resources
There is no risk Activity2 might accidentally reschedule Activity1?

If Activity 1 comes first (in priority and / or WBS) there is no chance that same resource will be booked first on activity 2.

As a completion.

Planning in ProjeQtOr is quite intuitive.
If fills availability as you would to it yourself, filleing cells in an Excel sheet, one by one, woith linear rule. :cheer:

Forget all the ununderstandable planning results you could have with MS-Project 👿

Activity1 is a fixed date (can't move) because for example we've arranged it with 30 people, a big meeting and simply something that CANNOT slip in terms of delivery. It's a specified time/place possibly with people around the world.

Just record it as a Meeting :woohoo:

Not possible, as it's a development task. For example code completed, so now needs to work with external company to test on a specific date/time, which may result in some more planning being required.