The extremely simple answer here is
Get much more money. Ask for it today.
If you don't get it the day you ask for it, then the following day start aggressively job seeking.
It's that simple.
This question comes up on here a lot. I always find the question really surprising:
"I'm in a job where I am working incredibly hard and doing a huge amount of stuff."
The answer is just so simple:
"Man, I wish I was you! Get a fatass raise right now."
Footnote. If you are struggling with the language for how to ask for a raise, send the following email to your immediate boss and big boss
Hi guys. Thanks again for the incredible job here. Presently I'm doing four roles, each of which is way beyond my current pay scale. Based on this I'd need my salary to be $xxx,000 {exact figure} from the first of next month. Please note that this would still be saving the company, at the minimum, two other full time staff. Could you give me your thoughts on this and let me know this week?
Just to repeat myself the question literally asks,
What is the best way of requesting more resources and a pay level consistent to that of my responsibilities and value add
the email wording is
I'd need my salary to be $xxx,000 {exact figure} from the first of next month.
plus add some polite buffer on each side of that phrase.
Always be extremely polite.
Keep it short.
Note - never, ever give rationales or reasons when you state you want a higher salary. The person on the other side of the negotiation is a professional. They will demolish your reasoning in two words.
For example, if someone told me
Similar roles outside of our firm pays 50% more than what I get
I'd just chuckle, respond with 13 words ("I just reviewed the market and you're making far more than the market"). Then to annoy you I'd add 16 more words ("Actually the provisional raise you just received, I can see no reason for. We'll review that.") Then I'd ask you to please get back to work.
The people who manage for companies, are ****s. Their only role in life is to keep your pay down - nothing else.
If you try to give "reasons" to a professional negotiator on the other side, they'll unfortunately just destroy the negotiation right there.
state the money you want
if you don't get it, immediately job search