I don’t know how Genki’s prices compare to the average for beginner Japanese textbooks, but I know that Minna no Nihongo and Genki (which are the two most famous ones) cost about the same. Are they worth that price? Depends on what your criteria are, but they’re popular and recommended by both teachers and quite a few learners, so I guess people do feel they’re worth it? However, as people have mentioned above, the first volumes of each of those two courses are definitely meant for complete beginners, so it’s not surprising if you found the Genki I disappointing or seemingly too simple. You probably know quite of bit of the content already. It might still be worthwhile to check out Genki II though, price notwithstanding.
If you want a textbook that’s efficient (in my opinion, given how I enjoy learning languages – i.e. with lots of context, detailed breakdowns of sentence structure, and with as few grammar-only study sections as possible) and costs less than possibly even just one volume of Genki or MnN, I’ve got a recommendation for you – the textbook I used. Granted, I think MnN or Genki might cover slightly more grammar points overall, but I don’t think the difference is significant enough to matter. Also, there’s definitely an overlap between the grammar my book covered and the first few chapters of Tobira, which is an intermediate textbook, so it’s no slouch either. It uses around 930 kanji across 98 lessons, and the grammar covered is roughly N5-N4, maybe with a little bit of N3 grammar mixed in. It exists as an e-course in English. There may be some paper copies floating around online on second-hand book sites/Amazon, but those are the previous edition. The latest English edition only exists as that digital course; the French original also exists as a paperback, if you happen to speak French. Anyway, enough blabbering from me. Here’s a link:
This course used to be called ‘Japanese with Ease’. I don’t know if the official title has been changed, but you can try using that to find reviews or more information if you want. Creating an account and downloading the programme will allow you to try the first seven lessons (almost definitely too simple for you if you find Genki too simple) for free so you can see how their lessons work.
By the way, going back to price for a second: I understand that Genki and MnN both cost roughly €60 per volume. That means that to get to around a full N4 as a complete beginner, you’d have to fork over €120 for the two relevant volumes with either course. Assimil’s course costs €50 as an e-course, and it also covers enough to reach about N4. Plus, I think it’s more efficient for self-study than either Genki or MnN, so why pay their price?
If you want a detailed breakdown of why I like the course, along with a picture of part of lesson 45 of 98 so you can see what ‘midway through’ looks like, here’s an old post from me. I start with the flaws, and then go into what’s I think makes it great:
As you’ll see in the photo, every single lesson comes with full parallel translations of what’s written in Japanese, and the earliest lessons will also come with full literal translations so you know what the Japanese sentence literally says and can compare it with the natural translation they offer. There are also footnotes pointing out important details that pepper the course. (If you’re interested and need me to translate the footnotes into English, just ask. Shouldn’t take me too long.)
EDIT: OK, turns out they’ve got a PDF online with sample lessons for the French edition, namely lessons 1, 50 and 97. Take a look if you want. At the least, you’ll be able to see what the Japanese is like at the end of the course too: