How to discuss workload and being too senior for certain tasks w/ PI

I’m a social & behavioral research coordinator, in my current position for 5 years. My PI and myself are the only permanent members of our lab. I am a part-time worker (although not far off from full-time) and I requested that schedule when I was first hired because I’m chronically ill and found in previous positions that I can’t work a full-time schedule and manage my conditions at the same time. I did not disclose my illnesses at the time (tried that before in a different industry and it didn't work out well for me), although my PI now knows about one condition.

I’m currently working on 3 studies, 2 of which I lead and 1 of which we’re relying on a collaborating institution for as the prime. For the latter, we’re doing weekday and weekend data collection that requires specialized training. Currently, two grad students, the research coordinator from the prime institution, and myself are the only ones trained to collect data. The two grads and myself have been doing the majority of the data collection and the research coordinator from the prime institution will step in when necessary.

We have a rota to schedule the weekend data collection and the expectation is that the two grads and myself will take an equal number of shifts and then we make up for those hours by taking a half day or full day the next week. The only problem is that, as a research coordinator (and essentially project manager because my PI isn’t doing that, although I do get assistance with a good portion of this from another staff member in our department), I have a heavier workload than the grad students and there’s often not time for me to make up those weekend hours, which means I essentially work an extra day when I do a weekend shift and then don’t get compensated for it.

I want to ask my PI if the plan is for me to do the weekend shift throughout the data collection period, which will last 3 years from February. I’m eligible for a promotion to project manager soon and my PI has told me he’s recommending me for it. I definitely don’t think it would be appropriate for me to be doing weekend data collection on a regular basis as a project manager (really any data collection, but needs must and I’m fine with doing the weekday collection regularly if the other tasks more aligned with my job description can still be done on time), but how do I ask my PI about this without it sounding like I’m complaining? I believe I’ve paid my dues—I’m almost 40 for god’s sake—but how do I say that without sounding snotty?

I’ll be honest—I’m pretty sure I’m burnt out. Part of that is being chronically ill and neurodivergent and I need to find ways to manage those symptoms so they don’t hinder my work too much, but part of the problem is having too much work for my part-time schedule. This one study isn’t the main source of the problem—but working on the weekend is making the problem much worse.

TL/DR: I’m a research coordinator (with a promotion to project manager coming soon) who works part time (but not far from full-time) and am the only permanent member of my PI’s lab. I’m currently doing weekend data collection with two grad students on 1 of our 3 studies, but my workload is too heavy for me to make up those hours during the week by taking a half or full day off like the grad students can, so I’m essentially working an extra day unpaid. I believe I’m too senior to be working on the weekend (or will be when my promotion project manager goes through) when we have grad students who are also doing the data collection. How do I talk to my boss about this without sounding snotty or complaining?