Setup secondary redundant failover Node.JS server - node.js

Question

Introduction
So I made a discord bot and hosted it on Heroku server. The bot is basic, it just listens to new messages and responds to them.
I am using a free tier of heroku and discovered that there are so called "free dyno hours". When it is end of the month, all the free hours are consumed and my bot will go offline until next month.
I own a Raspberry pi and so I thought it would be a good idea to host it there as well in case Heroku goes offline.
What I want to achieve
Host discord bot on hosting platform(Heroku).
use my Raspberry pi as a failover server.
Ensure that only one instance of the bot is running at a time.
If both servers are online Heroku(chosen as the main server) has priority.
If the application on Heroku somehow crashes or goes offline the bot on my Raspberry gets activated.
And vice versa.
How would that work if I had 3 and more servers?
How would I approach this if I had the bot connected to database?
This is just an example I would appreciate if more general solution would be provided.
NginX(load balancer)
When searching for help, I came across load balancers and NginX.
If I am right, it basically functions like a gateway, all requests go to the server NginX is hosted on and then they get redirected to one of many servers(depending on how loaded they are etc.).
I have few problems with this:
my bot doesn't receive any requests, it just listens to the discord api.
if the server with Nginx fails it all falls apart?
My solution 1
My first approach was to let the bot running on both Heroku and Raspberry at the same time.
It guarantees that the bot runs on at least one server if the other one fails.
But it is not ideal as the bot responds 2 times when both servers are up.
My solution 2
Heroku:
I added a simple Rest api endpoint with express.js to the bot app.
This way I can check if the bot on Heroku is running.
Raspberry pi:
I wrapped the bot app with this Node program that basically functions like a switch.
Link to a git repo: https://github.com/Matyanson/secondary-node-server.git
Every 3 minutes the program calls the endpoint and then by looking at the status code it checks whether the app is down.
Then it either starts the app or shuts down the app on raspberry(using pm2).
This kinda works but I am sure there is a better solution!
There is also a problem with Heroku(you can skip this):
Heroku uses Dynos(containers) to run the apps. There are different configurations of dynos including 'Web' and 'Worker'.
Worker is used for background job and never is never put to sleep.
Web dyno is the only dyno to receive HTTP traffic. It is put to sleep after 30min of receiving no web traffic.
I can switch on/off either of these.
both: there are 2 instances of my bot.
Worker: Can't connect to the endpoint.
Web: have to 'ping' the api at least every 30min or the bot sleeps.
Why am I asking?
Not because I need to solve this exact problem but because I want to learn a good way of doing this to use it in the future projects.
I also think this would be a good way to not 100% rely on external Hosting providers.