What books have you tried so far?
Most of my suggestions for other manga similar to Haikyuu level is going to be other shounen/shounen jump manga. I started reading things like Naruto, My Hero Academia, Bleach, etc., at around the same time I tried reading Haikyuu, and they all followed the same learning curve (where the first few chapters seemed more difficult and everything since then has been very manageable except when someone starts monologuing). Something like Attack on Titan is more challenging, but not significantly more so, in my opinion – you just have to look up more words since there’s more text.
On the more shoujo side, I’ve read and would recommend「不可解なぼくのすべてを」 (a beginner book club pick), 「明日、私は誰かの彼女」 (something Amazon JP recommended to me that just seemed interesting – it’s about rental girlfriends from the girls’ perspectives), and 「トニカクカワイイ」 (a cute story about a very young married couple dating and living together). All of these have been about the same level, though the first two don’t use furigana, and have presented a similar level of difficulty as Haikyuu (though in some ways they’re a little easier to pick up since there aren’t very specific sports or fighting related terms). I’ve also picked up ホリミヤ, and a quick peruse through it tells me it’ll probably be around the same level.
Books chosen by the beginner book club and rated at around 2.5 - 3.5 should be at around the same level as well, as should most of the books already read by the beginner book club, if you want to take a look through their archives.
～～Unasked-for Advice Alert～～
In general, I’ve mostly been picking up whatever manga seems interesting and giving it a shot. If it’s too hard, then I put it down for a couple of weeks and continue working hard at everything else, and then come back to it. It’s important to read material that will keep you reading, and that’s usually stuff that interests you.
I’ve also found it important to frequently push/challenge myself so I can improve. This can take on many forms, though it’s usually “read a little bit more” or “read a little bit longer” for me, and only occasionally “read something a little more difficult”. The jump between different levels can seem like a lot, but it’s easier to handle when your speed and stamina have been built up. For example, after about a month of (almost but not quite) daily reading with the summer reading club, I’m finding that I can slowly (start to) read an intermediate book club (light?) novel.