While @Jarod42 's answer is the right one for the given problem, I found a new way to approach the problem using C++26's expansion statements:

The unnamed structs should be tied in a tuple. We'll use a static_assert to check that all constants in the enum is associated with a type. Then we can use C++26's expansion statements, i.e. template for to loop through the tuple of unnamed structs:

static constexpr auto objects = std::tie(foo, bar, baz);

// Validate that all object types have corresponding parameter structs
consteval bool validate_all_objects_defined()
{
    
    for(uint8_t type_val = 0; type_val < MyType::TYPE_COUNT; ++type_val)
    {
        bool found = false;
        template for(constexpr auto& obj : objects)
        {
            if constexpr(myrequirement<decltype(obj)>)
            {
                if(obj.type() == type_val)
                {
                    found = true;
                }
            }
        }
        if(!found) return false;
    }
    return true;
}

static_assert(validate_all_objects_defined(), 
    "Not all object types have corresponding parameter definitions!");

template<MyType servo_type>
constexpr auto& get_instance()
{

    template for(constexpr auto& obj : objects)
    {
        if constexpr(myrequirement<decltype(obj)> && obj.type() == servo_type)
        {
            return obj;
        }
    }
    
    // This should never be reached due to the static_assert above
    std::unreachable();
}

This way, the users only have to add a constant to the enum, define a structure and add it to the tuple, and adding a constant without defining a new type gives an explicit compile error:

<source>:63:43: error: static assertion failed: Not all object types have corresponding parameter definitions!
   63 | static_assert(validate_all_objects_defined(),

Complete DEMO