Manager wants to hire more devs for the sake of growing the team (I feel it's unwise)

Unwise for you or unwise from a business perspective? What is the ROI for expanding the team? What are the future business goals? Does shortening the time to market if new features represent an opportunity to secure market share?

Odds are this manager is not proposing hires just to arbitrarily make the team bigger. People do silly things so I won’t rule it out completely but it’s highly unlikely. Odds are there is a reason that is justifying the expense of investing in a larger team.

I work at a very small company as the sole developer.

And there’s a good reason right off the jump. Your company has a “bus number” of 1. That’s not good. A bus number is the amount of people you can loose on a team (as if hit by a bus) before it represents a risk to the business. To put it lightly if you get hit by a bus, get sick, get a new job, decide to retire to cobble shoes, or leave the company got any reason they will now have exactly ZERO developers. And replacing you will be a challenge because as you point out later you’re a full stack dev with project management responsibilities.

Understand that adding single redundant worker to back you up and mitigate flight risk alone could be worth the cost.

The manager, who I believe wants more to manage, has been mentioning growing the team by hiring more devs.

This is also very possible. A manager who only manages a single person isn’t useful. And from the way you describe your output you don’t need much management. So really this manager finds themself in a role where they are an overpriced admin. That’s a dangerous place to be with the industry being as it is.

Here’s the thing though. His insecurity is not a business justification for the salary of several devs. So even if that’s why the manager is personally pushing for expansion that isn’t the reason the business is approving it.

I have 7+ years as a dev at various companies and I'm telling him we absolutely do not need any more devs.

You don’t need more devs to continue operating with current business goals. There may be future plans being discussed that are limited by the size of the team.

I'm constantly praised for having us way ahead of schedule and I'm doing (mobile, frontend, backend, cloud and security)

Good for you. But you’re still just one dev. If the company wants to expand and grow you may be the limiting factor on those plans. And given your staunch opposition to any level of expansion it seems likely that’s how the business sees you. You are good at delivery as an individual contributor, but you are a limiting factor holding the business back from exploring opportunities to increase functionality, market share, or scale.

There's really not enough work for a second dev.

Not enough right now.

Just enough to keep a nice list in Jira for myself.

Parkinson's law: "Work expands so as to fill the time available for its completion"

The inverse can also be true. Work contracts because goals have to be discarded because they simply aren’t possible at current capacity.

He's also changed out meetings so I almost never talk to the guy who is funding this project. I'd like to make my case as I think he would side with me.

Yeah, I’d do that too. You’ve already decided you’re against expansion without even bothering to ask about the business justification. You’ve also clearly indicated that you’re happy being an individual contributor so there’s no reason to include you in expansion discussions.

If it occurs, then after the expansion you won’t be moving into a team leadership or software architecture position. You’ll be kept on for knowledge transfer and to continue as an IC.

If the team expansion doesn’t occur your role hasn’t changed at all.

So there’s no reason to include you in any discussions. Regardless of outcome you aren’t affected. The best use of your time is to remain on task pushing out features and fixing bugs. They’ll let you know when a business decision has been made and it affects you. Odds are it will be something like “start making tasks to update documentation and make a list of onboarding activities for new devs.”

My fear is they will hire someone who will be vibe coding and hammering out tickets and we will run out of work or it will become a nightmare resulting in me crashing out and quitting which I don't want as this project has been deeply personal for me.

Your feelings are irrelevant to the business. And your unwillingness to even consider the idea that the company might want to accelerate development is probably why you’re not invited to meetings.

You can take shots at vibe coders (lord knows they are justified) but “hammering out tickets” is exactly what the company wants from its developers. It wants new functionality out in production increasing revenue as soon as possible.

Take a deep breath and really reflect on the truth: You don’t own the product or the code. It’s not yours. One day it will be taken away from you regardless of the reason. You may have an emotional connection to this project but it doesn’t have one for you.

What would you all do/say in this situation?

My goals are not your goals. I haven’t been just a dev in over a decade. If I knew the company was looking at expansion I’d want to ride that wave into a leadership position. I’d want to know what the ROI justification was and I’d start brainstorming ways to increase that return.

How can I tell the manager I have experience and I know this is unwise right now?

You don’t. If you had the kind of experience and expertise where you could link business objectives with technical solutions you wouldn’t have this attitude about the prospect of expanding the team. Your justifications are all about your comfort and emotional state. That has nothing to do with what is right for the business.

You said right in the title of this post “I feel it is unwise.” Your feelings are irrelevant. If expansion is wise or unwise isn’t a feeling. It’s math.

• Product X is not possible with the current team of 1.
• Product X can be unlocked as possible with a team of 5, and minimum viable product (MVP) is expected to take 15 months to limited production release.
• Product X mvp with a team of 15 is expected to take 10 months to limited production release.
• Product X is expected to be self funding by month 2 of limited release in production.
• Product X v2.0 is expected to take 5 additional months with a team of 5.
• Product X v2.0 is expected to take 2 additional months with a team of 15.
• Product X v2.0 at current market share should represent $1.4 million in revenue the first quarter at full scale release.

That’s what dev team expansion discussions actually sound like. There’s no “I feel”. There is a very good reason you aren’t involved in these discussions.