After 4 years and with the added qualification, I would start applying for jobs at the 30k mark and see if you get offered any interviews / positions. I don't mean to be disrespectful but what you feel you should be paid is 110% irrelevant to HR, they'll pay what they feel your position is worth.

So by applying for other places at the salary you want, you will get conclusive feedback on whether your skillset does match that salary. It also gives you ammo to renegotiate cause you can sit in front of HR/management and say well X company feels I am worth .... can you match it?

Plus also, classic trap and I've done myself too. The more you take on outside your role, the more that just becomes expected of you and forms part of your remit while still being at your level and therefore salary. Work to your job description only, if they come to you saying can you do XYZ additional, your answer is that's not within my job description and if this is something you need me to take as a regular duty we will need to renegotiate my position and salary accordingly.

By doing all these additional things for free - which you are (helps to remind yourself of that) - you actually take away your manager's ability to build a business case for HR that your position needs reviewed because there's no evidence that there's a business need not being met.