Mini split confusion (efficiency and sizing)

I'm right on the edge of buying a mini split for our shop space and I'm having a bout of indecisiveness.

To preface, we are located in south/central Texas. The shop is a standalone structure, is fairly well insulated but has a 2-car garage door and a single-pane window, so it isn't ideal. It's 625sqft with 9 foot ceilings. We have various tools that will be in use but nothing that creates excessive heat.

We are quite limited in funds so I'm not going with an HVAC company, nor am I going with a mr cool or other diy friendly unit due to cost. I'm capable to do the install myself, I'm not overly concerned about that.

I was originally looking at the 18k (1.5ton) senville leto. I ran various calculators and they all suggested this size would be enough up to 1000sqft or so. However, since I have 9 ft ceilings and I live in a hot climate and I have the big door and crappy window, the calculations pushed my 625sqft up to around 900 or so equivalent sqft in some cases and in others as high as 1200.

Due to my circumstances, I think I've convinced myself to move up to the 24k unit instead. I don't think it's too over provisioned for my specific scenario but I would like to hear your thoughts on how off the mark I am here.

Apart from size, there's also the question of efficiency. The other model I was looking at is the Aura by senville, and the difference is a higher seer2 rating and energy star certification.

Now, doing rough math, the difference in efficiency would be paid off by the savings of the more efficient unit in something like 8 or 9 years based on projected usage and my energy costs. I'm a pessimistic person and don't expect this unit to last 8-10 years. This is a cheap way to get the space comfortable to work in so that my side business can progress, and I would think if it's still going in that amount of time, I'll be able to afford a nicer unit by then.

However, after saying all that, I do notice that the capabilities of the Aura unit are greater. It is advertised to work in -20f weather while the leto works in as low as 5f.

Now, being in Texas, my primary concern is cooling, not heating, but no where can I find if the Aura system is more capable in hotter weather like it is in colder weather. This is slightly concerning because it seems most people use these primarily as heat sources and secondarily as cooling sources, and my situation is reversed.

Does anyone have any input as to whether the higher efficiency system will make a noticable difference in cooling performance when it's 110f outside?

Thanks for weathering my wall of text.