development process – Question about experience and pay

I really need general consensus between experienced developers on my scenario below. I’m sorry it’s not related to an actual coding problem.

I have a situation at a small company where the only developer was fired. My position was a field technician and they wanted to move me into a hybrid role as a field tech & full stack developer. This didn’t seem official because there was no pay change or paperwork changing my job title or description.

They’re now saying that I can become a senior full stack developer by developing a new program and some of the code from the old app to a newer app I’m creating in WPF. Also, migrating some old database they use for quoting & PO’s to a newer version that can be tied into their portal/website.

Not only did they not raise my pay, they didn’t even officially make a lateral move with paperwork with my position. Not only does this seem sketchy, it seems impossible. I have a diploma in programming & an advanced diploma in electronics engineering. I don’t have any experience in either of those fields other than maybe some transferrable skills from manufacturing facilities as a floor worker.

I don’t find it possible to go from being a junior to senior level full stack developer in only a couple years (or however long it takes me to code them the 3-4 apps they need me to create). Maybe it will take me 4 years, then it makes sense that I’d be considered a senior level developer (I’ve read some articles and they mentioned it would take at least 4 years to get this kind of experience in both front & back end programming).

Please give let me a hand as I don’t know how to negotiate or understand where I fall on this spectrum.