"Balanced" is just a relative term, can't really expect real balance between Taiwan strait, never was any balance and tbh. never will be, nobody is taking the word for it.
For now quality wise except for submarin fleet it's more or less balanced between Mainland and Taiwan, quantity wise there is nothing Taiwan can do about. Problem for Taiwan is in 10years or so, this quality balance also will likely tip greatly in favour to Mainland China, it's unlikely that any EU nation will want antagonize China with a major arms sale to Taiwan, so US would likely to still be the only source of military equipment Taiwan hvae in the forseeable future, so let's count what US have on Air force first, F16, F18, F35, F15, F22, unlikely for US to sell F15 longrange strike eagle to Taiwan since they seems to want to keep the arms sale to none offensive weapons as we have seen in the past, F22 is out of question for the same and alot more other reasons
, that leaves F16, F18 and F35, the F16 is the most likely one, as Taiwan already have the MLU variant, logisticly also easier, F18 is also a good choice since Taiwan is a Island, twin engine plane and relative shorter take off is wanted, F35 is more of a strike plane than air superiority fighter, but the platform is more stealthy and there is Vertical takeoff variant, comes in handy when all the airfield and even the highway in Taiwan is heavily bombed, but I doubt F35 is availible for taiwan anytime soon even if US approves the sale, there is a long cue of JSF project participating nation that is waiting for this plane, proberly availible after 2025, no sooner, when Taiwan really gets it approved and produced and get it combat ready it will be almost 2030, so before that Taiwan will be stuck with 3rd gen fighter, F16 or F18, with upgrades of course, but with Mainland China coming J-12 or whatever it will be named, as early as 2015 Taiwan need to face a Generation gap in AF.