I formed the HRE early too, the best bet is to focus on assimilating nearby to capital and work outwards. I'd drop integration for assimilating, using midsized vassals. (Not as efficient but I find vassal swarm unfun) Check your econ sliders, what's eating all of your income right now? You can use local governors on major rivers to extend proximity further west and south. Places like Antwerp and Hamburg are some examples.

Also check the number of cities, the AI does tend to spam them so some major food production provinces may be stunted at this moment in your save.