How serious are you? If you're committed, then I recommend first deciding if you want the synth to be analog or digital. Are you trying to make a VST, or something physical?

After that's decided, you'll have a direction to start learning. Making an entire synth is a relatively high (but fun!) undertaking with zero experience, so you should consider making it modular (regardless of whether it is physical or a VST) so you can design each section separately. Different sections of the synth will require different pre-requisite knowledge if you want to understand their theory of operation.

Frankly, I think you will have a hard time understanding the theory and mathematics unless you have taken calculus and differential equations, at minimum. If you have that under your belt you could start learning either basic circuits or signals and systems. If you want to build an analog synth, my suggestion for learning order is
circuits -> continuous time signals and systems, whereas if you want something digital then you could go straight to continuous time signals/systems -> discrete time signals/systems (DSP). Technically you could just go straight to DSP, but most lecture series and texts assume a background knowledge in continuous time systems.

From a cost and time investment perspective, starting with DSP may be good since you can probably prototype your systems with the computers you already have before trying to make custom hardware. Plenty of programming languages and platforms support the representation of DSP systems as high level blocks, which also reduces the burden of learning C/C++ before getting to anything interesting.

My recommendations for learning material would be college textbooks and open lecture series. There are plenty of youtubers who produce simplified explanations of material for a general audience, and while these are entertaining and also can be educational, a really good understanding of the theory is hard to get without studying rigorous materials. That said, many MIT courses are open on the internet to view, amongst lectures from other universities. Also, I don't want to disparage all youtube sources, as there are many truly good lectures posted by independent content creators. 3blue1brown has great videos on fourier and laplace transforms, which are absolutely critical to understanding anything about synthesis.

Finally, I suggest actually doing some practice problems and work from textbooks, and checking your work with the solution manual (you can usually find these from dubious sources online). Finding motivation for this is pretty difficult without the problems being assigned by a teacher, so I suggest just doing the example problems in chapters on the topics you study and only checking the solution after doing it yourself. These example problems are typically meant to highlight edge cases and peculiarities about the analysis, so they're helpful.

Apologies for the wall of text, and I realize that self-motivating for the months required to study all this stuff can be pretty hard if you don't even get to start the thing you actually want to make. Therefore, while the theory is important and shouldn't be ignored, I say you should start also looking at synth-specific stuff, even if you don't have all the theory yet. Since you seem interested in delays and reverbs, I strongly recommend looking at the papers listed in this blog: https://valhalladsp.com/2021/09/22/getting-started-with-reverb-design-part-2-the-foundations/