That sounds a bit like the F-35, which was supposed to be a common platform for three different military jets with highly conflicting requirements (US Air Force wants an affordable single-engine stealth supersonic fighter, US Navy wants it operable from a aircraft carrier, US Marines wants it hovering like a Harrier), which is the definitive example on how to NOT manage a multirole jet fighter program, to the point where it's still not in full-rate production twenty years after the start of the program, arguably still not fully operational, still hideously expensive on per-hour fly rate and finally the US decided to both modernize old F-15/F-16 jets and start developing the sixth-generation NGAD.
The Dassault Rafale on the other hand has three variants (B for twin seats, C for mono seat, M for the French Navy) which are basically identical except for a reinforced undercarriage for the M variant. Increased development costs and delays were mostly because of shrinking defense budgets and political squabbling post Cold War, not because of technical development issues. While the F1 block was an emergency stopgap for the Navy, later iterations and especially the F3R have proven to be very versatile and able to carry out basically any mission that the French Air Force and French Navy has, to the point that it is expected to be the only fighter/bomber/interceptor type in service when all the remaining Mirage 2000 variants get decommissioned in the coming years.
I'd say the main differences are that culturally Dassault at its core is more of an design and engineering shop than an aircraft manufacturer (most manufacturing is outsourced, critical components and assembly are done in-house) so emphasis is placed more on design than production [0]; France wants an independent defense industry and an all-around fully capable military on a budget, thus we have no choice but to keep our costs under control in order to afford all of our gear; and finally while the Rafale ecosystem has a lot of companies (Thalès and Safran for the biggest subcontractors), Dassault is unquestionably in charge of the program industrially and the DGA knows better than to try and micromanage them or to ask impossible or unbearably expensive requirements.
[0] https://www.rand.org/pubs/reports/R1148.html for an analysis of the Mirage days of Dassault
At the end of the day, poorly implemented systems, etc are far worse than shared technology.
The biggest(?) example of sharing development to a greater degree is the F-35 plane. Three models - themselves mostly the same - were developed for about a dozen countries and more services (e.g., in the US, F-35s are used by three services, the Navy, Air Force, and Marines). The idea was that by sharing development costs, they could all buy better planes and then standardize parts (creating economy of scale there), maintenance, training, and functionality - everyone's planes would work perfectly together in combat, you could land your plane anywhere and get parts and maintenance, one country's pilots could fly another's planes. That's all fantastic for a deep military alliance.
But a big challenge is making three variations on one design please so many customers, fit so many needs. Think of development or integration projects in IT: Understanding, defining and meeting the specs of customers is often the hardest part. Imagine doing that for organizations as large as one US military service, such as the US Navy with all their needs and users and powerful people who want their way - imagine the specs of a plane that lands and fights off aircraft carriers - and then the powers in government that funds them, people in Congress, the White House, etc. etc. Now expand that to a dozen countries and all their military services - with different sized carriers, or different fuels, different cultures, different approaches to air defense, different legislators, etc. By serving all these needs, you can end up with outcomes that please everybody and serve nobody.
The F-35 has great positives - it's widely considered the best plane in the world - and negatives and has been the target of much criticism (including by people who didn't get contracts or get their way). Nobody seems anxious to repeat the process to that degree, but that doesn't mean it has no value.
They tried to maximize commonality between the 3 variations, but from what I can tell, this plan hasn't worked out, and they've ended up building 3 different planes that happen to look similar.
My suggestion was to drop the pretense of building the same plane for the French and the Germans - but use the same (or similar) engine for all planes, use the same or similar radar, and avionics, and put them in different airframes.
Afaik at this point, one only needs to reprogram the avionics with control laws for each specific airframe.
Then let each nation put their own sensors and bits and bobs to satisfy their own requirements.
From what I've read, the Chinese are sharing the engine, radar and avionics tech between the J-10 and J-20 fighters.