Hi, someone who made a complete solo SRPG using Unity once here - I’d rather not post the title since it’d essentially dox me (and it’s pretty bad too), but I can DM it to you if you want. Thought I’d share some of my experience with it, but keep in mind that I’m speaking purely as a solo dev (so if you have a team/the money for commissions it’s not super relevant) and I’m definitely biased towards using fangame tools (romhacking/LT/etc.) nowadays, so it’s probably more of a “I regret ever doing that, it’s a terrible idea” post than what you’re looking for. If someone who prefers going indie over making fangames could chime in that would be much appreciated I think!
(I wrote this from the point of view that you wanted to make an FE fangame and considering using Unity or not, just now realized it’s probably more using Unity and considering an FE fangame or not - sorry, FE fan, can’t read. Tl;dr wouldn’t recommend FE-like as a Unity project either, make literally any other genre)
Development Time
In terms of development time, I made it solo during university in my free time and it took me about 3.5 years to complete. It’s hard to gauge length in FE chapters due to some structural changes, but it’s roughly the equivalent of 10 chapters with 5-7 hours of playtime to reach the ending (around 15 to get all achievements and stuff). I haven’t made a full-length fangame yet, but I’m almost certain it takes way less time for a much longer campaign if the one month contests like CELICA or SIGURD are anything to go by (although input from actual full-length hackers/LT users would be appreciated here).
Content
Content-wise, it is significantly harder to make something from scratch over using a fangame tool - because most of the work has nothing to do with your game’s actual story or characters or chapters. With fangames there are so many vanilla assets/code you can simply use without a second thought, but going indie forces you to make all of these for yourself - and it is a huge amount of work. Seriously, I can’t stress this enough: it’s definitely not something I thought through going in and really burned me out by the end of it. Just fire up the prologue of FE8 and look at all the tiny details players take for granted - battle preview, scene transitions, movement range display, cursor movement, every single menu beep, etc. etc. - these are all things you’re going to have to draw and sequence and code and insert and debug yourself, and unless you love going into this sort of details it’s going to drive you insane. Not to mention the way more prevalent - and more time-consuming - things you can basically skip altogether making a fangame: UI, tilesets, conversation backgrounds, battle backgrounds, battle animations, and so on. Even if you think this sounds fun, this will be what you spend like 80% of the time working on, so make sure you are absolutely on board with it before starting I’d say.
Also, it is much harder to keep things consistently high quality when working solo from scratch. Of course, if you are a talented artist/programmer/musician/etc. this will let your skills truly shine free from the vanilla limitations - but if you have even a single weakness, it can and will be immediately apparent, whereas vanilla/F2U assets can easily cover that aspect for you in fangames. Another reason I didn’t post the title here is that the game is absolutely hideous in every single screenshot… But anyway, make sure you’re good (or at least passable) with all these roles and enjoy doing them (super important!), since you will need to make absolutely everything (or a have lot of commission money). Granted, some things like writing or game design are shared so uh, better practice these anyway.
One last thing: a potential motive - at the very least, it was a big one for me - for making an SRPG from scratch over a fangame is going beyond what vanilla is capable of, but honestly? You can do so much stuff with vanilla nowadays, not to mention LT, that unless it’s really out there like full 3D graphics you probably don’t need Unity for it. Your idea has a custom, non-vanilla campaign structure? Check out the roguelike Embrace of the Fog or open-world The Unbroken Thread. Want to have a unique artstyle? See every game by Not Quite Rodents like Homecoming. Is music your passion? Eislinn’s Last Sunset has a completely custom soundtrack with multiple themes per map. Personal bias here, but you can also check out some MARTH/CELICA/SIGURD entries to see the sort of super cool and unique things people can do with FE8 or LT - it’s really awesome!
Players
This is more up to personal preference and marketing skills, but from my experience game dev is a way lonelier journey. You basically have to constantly fight to get any attention whatsoever among the sea of other indie devs, and it’s really hard to find any players, not to mention feedback. I participated in Steam Next Fest, and got… two whole feedback posts. I was also lucky enough to study in a country with indie dev events for a semester, where I showcased my game and… maybe three people played it? Granted, I am utterly atrocious at marketing and communication, but still. You need to be incredible at marketing to get any feedback or players.
Over two years post-release, it has 14 steam reviews and ~400 downloads. And this is a free game - I’m certain that had it cost even one dollar it’d be like 40 downloads at best. Just as a point of comparison (sorry for shilling again): my biggest romhacking work, The Pen, currently has 181 downloads. And that’s something I made in one month compared to over three years.
I’m not saying that a fangame will be more popular than an indie SRPG - it probably won’t - but if you think like I did that publishing something on Steam will inherently let you find more players, that uh, doesn’t work. Really, really doesn’t work.
One final thing and a big advantage for indie dev though: it’s way easier to showcase & explain to irl friends/family compared to fangames, especially romhacks. I got pretty much everyone I know to play my indie game - but only one friend is familiar enough with what romhacking even is to try out any of my hacks.
Future
Publishing a solo indie project is an incredible portfolio point if you seek a job in game development (at least that’s what I liked to tell myself), although I know that some people also put fangames in their portfolios. But yeah, if your goal is more to get better with Unity/C# to eventually land a job in the games industry (or become a full-time indie), this would be an excellent experience and it does teach you a lot. The only reason I don’t regret making my own project to be honest (aside from finding out I hate game development and the industry in general).
Anyway, this ended up being longer than I expected and more of a rant, sorry! Hope there’s still something useful for you in there. If you have more specific questions feel free to reach out 
The game’s also open-source if you’re interested in more technical reference, but again, I made it during university so it’s an absolute mess. And good luck with your project!