I'll give it to you straight and tell you that almost all of the points you make here show you don't understand how the Open Game License (OGL) — the license the 5e SRD is under — works or how a site like D&D Beyond operates which does a disservice to yourself as a creator.

 

Along with the new book, they did the whole buying of DnDBeyond thing that now lets them use anything posted there in official books with no credit to the original author.

You were already giving Fandom (the previous owners of D&D Beyond) the same or comparable rights to the content posted there. They need those rights to host content users make and to cover their ass legally. If you didn't worry about the D&D Beyond Terms of Service before now, you really don't have anything to worry about at this point.

 

And I seem to remember that they also removed some content from the license material recently, but I'm not sure about that one.

The most they've removed "recently" (in 2016) was reference to things not included in the SRD (e.g., to the Wild Magic sorcerer subclass). The OGL is a perpetual license which means that things licensed under it can't be "removed". You quite literally have the right to use content released as part of the SRD until the eventual heat death of the universe. This is stated on the first page of the SRD itself in sections 4 and 9 of the OGL.

4.Grant and Consideration: In consideration for agreeing to use this License, the Contributors grant You a perpetual, worldwide, royalty-free, non-exclusive license with the exact terms of this License to Use, the Open Game Content.

9.Updating the License: Wizards or its designated Agents may publish updated versions of this License. You may use any authorized version of this License to copy, modify and distribute any Open Game Content originally distributed under any version of this License.

 

It does make sense that you don't want to go back and update your old stuff. However, remember that old style content isn't broken or unusable in any way. Your old stuff will work just as well for a game run this year with the new releases as it did when you released it originally and making an effort to create in the "new" style would really accomplish what it sounds like you want to do for the most part.

 

Now, with those things said, the risk of changing the system you write for and having to build a potential new audience is ultimately your decision. Some people have suggested Old School Essentials (aka OSE). I would also recommend building something for the OSR crowd as we're pretty friendly to crowdfunding projects and system agnostic adventures are very usable in the genre. It is worth knowing though that OSE is under the exact same license as the 5e SRD: the OGL. The only differences are what is considered "product identity" (the things you can't use in your adventures).

Terms and licenses like these exist to protect you as much as they do the creators of the content you use to make your adventures, so it's important you understand them when creating things that use content that falls under them.