So I have a few novels that I want to read where they have a manga adaptation (example: ReMonster). In regards to any novel that has a manga adaptation, is it better to read the novel from the beginning or just pick up from where I’ve left off? Reworded: I don’t wanna wait for the Tensei Shitara Slime Datta Ken manga to update, read the novel from the beginning or skip to where manga leaves off?
Almost always, you should read it from the start of the novel. If you're bored, then you can skim until you reach more intriguing parts.
Most of the time it's better to read the novel from the beginning. Manga often skims through events, skips some entirely, doesn't fully portray everything the same way... Even for the more faithful manga, pictures might be worth a thousand words, but do you really think every manga panel is going to somehow convey multiple paragraphs with perfect accuracy? ... And that's the more faithful. Less faithful manga make drastic, sweeping changes. Or even just weird minor changes. Like one character in the manga for BTTH became a kid for whatever reason.
As the others have said, is better to re-read. But if you don't want to, for Tensei Shitara Slime Datta Ken, at least read from the start of the current battle rather than after that lackluster megiddo.
I did read the Mushoku Tensei manga, and wanted to read the novel. I started in the novel at the point where the last manga chapter started. From reading that part I noticed that there was a lot more information in the novel that was left out in the manga. To not miss anything I readed the novel from the start, which I don't regret at all.
I am in strong opinion that novels must be enjoyed in its purest form But for some novel, it is not a sin to enjoy it in manga / manhua form I recommend BTTH to be read in manhua form instead of its novel, as 1 chapter of manhua contain 20 chapters of novels iirc and mainly every novel out there that contain too many unnecessary detail and somewhat a 'bland' narrating style.
Generally, it's best to start over from the very beginning as the different versions can have very different content. However, given the quality of some of these books, I don't really think that you'll miss much by just skipping ahead to where the manga left off. Sure, it's a different story if the original is pretty good like Overlord, but it's not as if books like Slime Tensei or Re: Monster are at that level. Hell, I wouldn't want anyone to read more of Re: Monster than they absolutely have to!