Many apologies if this is searingly obvious, but what's the equivalence of culture in colonization? How do I stop opponents from encroaching on my land with their borders? Do I need to add Liberty Bell professionals?

Many thanks!

Yes, liberty bells are culture... with a twist. :)

And the culture flip works quite well too; though it does seem to takes lots of bells (75% influence on the Native settlement tile?) so if you do it early there may be an issue later with the REF.