Hmmm, are you trying to express a degree of commitment in a romantic arrangement?
Or is this like a mathematical or scientific or legal thing, i.e. a strong relationship between gravity and tides?
If the former, I would give up 強い altogether and go with something like 真剣な付き合い (or any possible variation)
A quick search on google for "強い関係" shows that the term is used (87k hits)...but it seems to be used more in the context of relationship = correlation or relationship between two things/organizations rather than an interpersonal relationship. Here's a typical hit: にんにくとアンチエイジングの強い関係
The strong relationship between garlic and antiaging