Family & de facto law, financial agreements, consent orders
Okay, so you basically bought the property immediately before cohabitation (assuming that is the first time you lived together)
To answer your last question first, yes you have first option to buy out his financial interest in the place. What would his financial interest amount to is the key point.
Very basically, any of your own proceeds that you had prior to living together that went into buying the place is considered an initial contribution on your part... If your relationship is considered short (generally 5 years or less), then often the initial contribution is taken as belonging to that person... As the length of relationship lengthens, that can get eroded somewhat. At 8 years, you wouldn't expect to retain 100% of that initial contribution.
The norm is then that you divide by half any capital growth in the place.... Then adjustments for other things can come into play, the major one being children..
SO... what I recommend first off is that you attempt to resolve your issues & save the marriage. A good marriage counselor can often help a great deal. If that is not possible & you are heading for divorce, then that same counselor can help you talk about separation & property, & hopefully, you can both be reasonable & reach an agreement on how you want to proceed..
The best method is obviously one that involves little or no legal costs.. I recommend you have the place valued by a professional property valuer ( not necessarily a real estate agent because often they are not professional valuers)..
Then together, take all the facts & figures along to a family law solicitor to advise on what a fair & equitable split would be in your case. If you can agree on a figure, & you can afford it, then have consent orders done & buy out his interest.