Have you considered an earlier surface pro model.
First and second gen surface pro devices are cheap these days. I have a secondhand surface pro first gen device for less than 300 US dollars. Surface 3, pro 3 and ceratain pro 4 models are also worth considering secondhand or refurbished.
All surface pro are high end high quality devices.
I currently own two surface pro first gen devices. One I orignally bought at launch and one 3 years old second hand. Both are still in good condition, are snappy to use with general use. Batterylife is naturally lower with previousgen Li-ion batteries, but not per se due to capacity loss. I am convinced that surface devices are a good investment, even if a couple of years older. The quality is simply very good. The biggest trade off is battery life, especially when baked into the device and non-replaceable. Maybe some minor lagging in spec performance such as pen and cpu performance. But the i5 3317U chipsets still provide a snappy performance with windows 10 creators update. screen still performs well, nightlight works.
Batteries age as soon as they leave the factory. So a 4 year old OG Surface Pro has a 4 year old battery. It doesn't matter if it was forgotten and shrinkwrapped and tucked away. The battery is 4 years old. And as batteries age, capacity drops.
I still love my Surface Pro 2. But that's as far back as people should consider unless the original generation Surface Pro is dirt dirt cheap. For one thing, the OG Surface Pro has Ivy Bridge for the processor. Pretty ancient and poor with battery life even with new batteries.
The Surface Pro 2 has the same overall form factor, but made a number of significant improvements. For one thing, it has Haswell. And it has the 2-position kickstand. I tried out the original Surface Pro kickstand and I admire its owners because that one position is maddening.
Remember that the Surface Pro 3 was lined up to get Broadwell. However Intel was very late with Broadwell so SP3 ended up with the hotter running Haswell that came from the Surface Pro 2. The Surface Pro 2 was built like a tank and had a thermal design that could withstand the hot Ivy Bridge. That's why Haswell ran better on SP2 than SP3. Because SP3 thermal design was designed to handle the cooler Broadwell but ended up having to manage the heat from Haswell. So SP3 could throttle under load because of heat (which is why i7 on the SP3 was terrible).
My Surface Pro 2 with Core i5, 8 GB RAM, and 512 GB SSD still feels very fast so performance is not an issue. In fact, it's comparable (though slower) than my Surface Pro 4 with Core i7/8GB/256GB. And the SP2 is built like a tank. When you write or draw on the screen, there is ZERO flex. My SP4 flexes all the time because it's so thin and light. You can literally see the screen pinch as you apply pressure just like when you press against an LCD with your thumb on the laptop. On the SP2, you press down on the Gorilla Glass and it's like a rock solid piece of glass.