which is why you should stay at some ranks until you unlock multiple lines of ships
you need the fleet strength, exercise, etc. anyway to continue. takes a while to learn all roles, modules, and also have ships of different sorts just to be flexible.
with the new balancing higher ranks have less of a difference now, so actually they just tried to improve exactly that. if you take a lower rank ship in a higher rank game, you will have resistance and damage bonuses. I played a lot of r9 lately against r15, even against good pilots with good ships - you will not have a clear advantage, except your ship is smaller, but you will not suffer so much as it was before, when there was no thing like this.
if you go up further from a full house R1-R9 synergized, synergizing the second half of all the ships is going to be a lot faster, than going up only with a few. each ship you finish is one percent after all, and having the low ranks synergized vs. having the credits to unlock more ships should actually make it harder creditwise aswell in mid-progression, so i would still try to synergize lower ships more, while you already “unlock” some of the ships in higher ranks you are going to fly later.
another good tip is to safe up free syn to be able to instarank to syn2 (for the first bonus), or prepare higher tiered ships by going for your strong points, so one ship already awaits you with full syn and equipment for the hard games. so if you were a good styx in r9, go for minotaur in r11, etc.
going like this, you will spend some time in “medium” (old T2-T3), but snowball up to R15 in no time once you have a good buffer of credits. For people with less PvP strength, there is still the option to farm pves instead, but I rather would play pvp to get better in it.
btw. I understand “heavy” as “schwer” in german, which means heavy, and difficult, at the same time, probably also in other languages, but in english i think it’s rather “difficult” or “hard”
with the new larger tiers (easy, medium, hard) it’s also easier to rank up multiple lines of ships, just make sure you get everything to the last rank of your difficulty, before you move on. your best R9 ships should of course stay with you, while you start to unlock R10+. The difference between R9 and R10 in hard is minimal.
As a sidenote: larger tier groups basicly reduced the problem between ranks from 3 ranks per tier for 5-7 queues to 5 ranks per tier for 3 queues. So this gap was always a bit there, but now, while it gets a bit bigger per tier, there are actually only 2 gaps now, which thankfully to the bonuses do not count as much as before. So exactly this problem you describe has been improved lately, and even if they would not have unified the tiers, the bonuses would still make sense - and the difference between ranks was still prevalent, basicly negating the usage of low rank ships in any tier.