While your post does mention notifications which really helps with engagement and was lacking from most forums, the main issue was IMHO lack of good mobile support of all the main forum platforms until as you said Discourse came along, but by then it was too late.
You might want to first identify your main power consumption and at what time this occurs. 300$ seems very high in general, but if that is for example mostly AC usage during the day, you might be able to not have to buy as large of a battery since peak consumption coincidences with peak production.