Streamline FT monitoring code

• avoid partial construction (construct + later initialize) by prefering to delay construction to the point where the objects can really be constructed
• do not template if not needed
• prefer std::array over std::vector
• consistently use fmt::format to format text
• use using to avoid repeatedly spelling out the same long template typenames
• fix use of std::shared_ptr in one case where the shared_ptr was not kept alive while its payload was being used