We've been full remote working here at In Excelsis since the beginning. We had to face those challenges and I find Holacracy was of great help for precisely separating the "work space" and the "tribal space", the latter being one that should definitely be adressed! The way we did it is a lot of inspiration from Jason Fried's "Remote" book, along with trial and errors and course corrections.
After three years, we faced the same challenge you described, despite the monthly "journées INXL", the occasional lunches here and there and a lot of online meetings. We envisionned two paths: renting a co-working space for those of us near Paris, or renting a "company house" yearly for sprints together every other week. Life provided us with the first option and we then dismissed the second one (my preferred option, but well... :-) ).
We're pretty happy with it, even though it's just been a month and a half. It definitely brought back motivation for some of us. As for myself, I lost a lot in personal productivity (nothing beats long hours of uninterrupted work) ; but communication is better, and I see motivation is naturally "rejuiced" each time we see each other (even when it's only twice a week considering our current engagements with clients).
Now, I wouldn't consider co-working as a waste of resources ; far from it! If you find a place where there's a real community and not just brunches, nice colorful sofas and people sitting next to each other, you can indeed find tremendous value in it in terms of networking, expanding your reach to other POVs and companies, and even access to resources you couldn't have had otherwise.