I'm a mobile developer as well, and have fought (and both lost and won) this battle several times before. I've always focused the argument on quantifying the financial benefit to the company by pointing out how much a slower Mac costs them, specifically how slow compile times reduce productive development time.
For this I estimate how many minutes a day and hours per month I'm waiting for compiles to complete, and present it as lost development time. Now, the counter-argument is that you can spend that compiling time doing other productive things such as replying to emails, documentation, code review & checkin, etc. But the counter-counter-argument is a faster Mac makes you faster at the rest of your work as well, but it's just hard to quantify. As a former manager, I'd also point out that recruitment is a little bit easier and turnover a bit lower if the company is good about giving developers the best computers and tools, but again a tough benefit to measure.
The cost of a brand new top of line MacBook Pro (say $3,200) is less than $100 a month in depreciation and interest (typically Macs retain at least 50% of their value over two years, so that's at most $800 a year in depreciation). My cost to the company is roughly $100 per hour (take your pay, add 7.5% for payroll taxes, and costs of benefits like medical, etc, then divide by total hours a month (not including paid vacation days). So I only need to save 1 hour of compile time to justify the purchase of a brand new MacBook Pro every couple years.
Also, you should consider whether this is a company you want to stay with. I always advise young developers to interview for other jobs every year or two, because as you gain skills rapidly most employers have policies that limit how much they will increase your compensation to match your burgeoning market value. And you might find another job that's not just more lucrative, but at a place that has a better attitude about equipping their developers with modern tools, and more enjoyable and challenging work.
Finally, you might also wait and retry when Apple releases high end Apple Silicon MacBooks. I'm doing my development on a 16 Gb M1 MacBook Air and it's extremely fast, building my project faster than any Intel MacBook, and faster than almost every high end iMac and some iMac & Mac Pros. But it is limited to only a single external monitor and 16 Gb of RAM. When high end Apple Silicon MacBook Pros ship later this year, it's almost certain they will support more RAM, multiple external monitors, and be even faster yet. You should look at requesting one of those, I will be buying one, and selling my MacBook Air after less than a year, because lots of RAM and two big monitors is important to productivity too.