It looks like you are off to a good start. Are you doing your MAcc at the same school going from Spring 2021 - Fall 2021?
I went straight through undergrad, to a MAcc and on to a PhD. I think it works well, but work experience can definitely be helpful. If you end up doing anything related to audit, work experience in audit is very nice. Same is true for tax. If you look at financial reporting / disclosure work, it isn't as necessary to work first. My thinking was that I knew I wanted to go into academia, so why delay. Most people will go for Big 4 work experience, but I have one friend that worked at a mid-size public accounting firm and was able to get broader experience in two years than he would have at one of the huge companies. That can actually be a good way to go too.
Do well on the GMAT. I think there are still returns on getting a higher score all the way to the top, so study a bit and do well. If you are testing around 750, aim for that or a bit higher.
Courses for the upcoming year: I think the most useful courses I took before the PhD program were the accounting PhD courses I took during my MAcc. It sounds like you are at a PhD granting school. See if you can take the PhD seminars as electives for credit during your MAcc (this is what I did) or sit in on them. Keep in mind that they are at least 3 times as much work as a normal master's level class. This is also a great way to get a good and meaningful letter of recommendation.
Next you have methods courses that you can worry about. These are things that will prepare you for the economics and research-type thinking that you do in a PhD. Calc 3 is probably next on the list, along with linear algebra. Both constitute basic math classes that you need to do economics at a graduate level. Next, I would go for an econometrics course. This will be very helpful in understanding research papers and statistical inference. Probability and Statistics from the math department are useful to take. These take a calculus-based approach to statistics that you probably haven't seen. (I'm assuming your stats courses so far have been from the business school. These are quite watered down.) Beyond that, you could go for an advanced microeconomics class and an intermediate macro. There are more math and econ classes you could take, but I've already listed quite a bit and anything else would go beyond what I have personally taken.
If you are at a research school, it is common to have outside speakers come in and present working papers. This is a great way to see what accounting research is and how other academics critique it. Obviously, flying people in is different right now, so this might be happening virtually. Check in on these and attend if you can.
Research Experience: Work as an RA would be helpful. I think the PhD courses and the colloquium that I mentioned are actually the correct start on research experience. It is hard to get research experience without exposure to research. Getting an idea of the current research and being able to express what kind of research you want to do is a good start for admissions.
Letters: Some of the things that I have mentioned will help with letters. Having been a good student, should get you decent letters as well. RA experience will also help. I would suggest getting a letter from the best known person that you can. It is tough to know who that is sometimes, but it is usually some sort of endowed chair professor at your school. If you want to message me and tell me the school, I can give you my opinion on who that might be. Of course, a letter with really good content is better than a boilerplate letter from someone good. All else equal, a bigger name is better.
I think that is about it for now. I'm happy to answer more questions that you might have. Be sure to reach out to faculty at your school for advice too. They might be able to get more specific.
Also: Check out this site if you haven't - Phdwiki