The unknown to me is what 'ready for production' means. On reflection I think that since DRDO and ADA are responsible for designing AMCA, they cannot know when all will be arranged for production to start. HAL, given the job of building prototypes and testing them, can estimate how long that will take. Unless GOI negotiates and gives a production order in time for production facilities to be ready, poised to start operating the moment testing finishes, there has to be a delay between testing finishing and production starting.
The idea that there is political or labour issues is incorrect unless the government is formed by foreign funded parties. If the government is by Indian parties without foreign hand, then this won't even be a problem.
If DRDO develops the technology, it will give ToT to manufacturing organisation and handhold for some time to resolve teething issues. So, it will be production ready immediately the tech is developed.
The current problems in LCA production is mainly because:
1) Several key tech which were in parallel development to design have not matured & faster production means more production of LCA with higher import % which will in turn cause import dependency in long term for spare parts
2) LCA is not preferred due to size being too small. It was conceived as a replacement of Mig21 as point defence plane but with increased Indian strength, India wants more offensive power & LCA is not suiting the needs. So, LCA is produced as a stop gap measure to get data, feedback, gain expertise & act as testbeds for better planes - MWF (LCA Mk2), TEDBF & AMCA.
I don't see any point in India considering Su-75. As far as I know the design of Su-75 is fluid, with Sukhoi still conducting research into what design would sell in the international market. So far no real demand for the aircraft has been identified. Since it is a private venture to design a low cost 5G aircraft for export, Russia's pariah status for much of the world makes it a non-starter in my opinion. In addition, I have the impression that co-operating with Sukhoi on FGFA did not go well, so why go down that path again?
If India has the technology to develop the systems to go into AMCA, why not just stick to that? Which raises the question: can India develop AESA, EOTS, IRST, MAWS, sensor fusion etc to go into AMCA Mk1 and productionise the same in time for the fighter to go into production early next decade?
India is already developing AMCA with design being nearly finalised. It makes no sense to go for another development of Su75.
As for sensors, India is already making serious progress in sensor development, most of which will be implemented in Super Sukhoi & LCA Mk1A program. Additional development like sensor fusion, more advanced tech etc will likely be developed for MWF, TEDBF & AMCA by 2030s