Also, an RPG is usually VERY content heavy. Maybe that’s your thing, though.
If it’s a tactical style, the thrill is in the tactics more and the amount of content could be reduced somewhat.
In order to make the story compelling, don’t forget to ask the question, “Why?”
Why is this guy trying to make a time gate? Why are all these things happening? Why does he want to go back?
The answers may seem simple or trivial, but actually the answers to those questions define who the people are and what the stakes are in the central story crisis. Without asking those questions and giving relatable answers, your audience will have no empathy for the characters and ultimately feel uninvolved.
What is the flaw of the character he must overcome to accomplish his goal? Often in RPGs it is the very tried and true (yet boring?) answer, the character is too weak/inexperienced and must gain more experience. If you want me to care maybe it should be something more.
In Back to the Future, Marty has two passionate goals he must achieve. His own existence depends on his Mom and Dad falling in love (which he inadvertently interrupts) and he wants to save his buddy from getting machined-gunned to death (something he witnesses before he travels back in time.) Pretty strong stuff.
In other time-based stories someone wants to go back and change something tragic like a fiance dying in an accident. Something that could be so easily overcome. These are the types of things that make your audience/player care.
In this story, you have the main character, but you also have this clash of tech. I assume the high-tech stuff is VERY limited in number because medieval weapons really are no match for mecha, no matter the siege weapons. Watch a film like Nausicaa of the Valley of the WInd to see a GREAT mesh of clashing tech. In that world, there are remnants of tech from a destroyed world. So, there are occasional technical wonders mixed in with masses of slightly older tech like guns and tanks and then armor and swords.
Already I see the scope getting carried away again. If this is one of your first games… why not something like Portal? Much more limited in scope, but beloved.