For what you describe, you should absolutely keep using your current laptop with a new battery and SSD drive. It is beyond simple to replace them, have a look at iFixit's website. Batteries vary in price from £30-£100. I would definitely get a high end one so you get proper power and trustworthy build. I hear NewerTech/OWC are good. For SSD hard drives consider the Samsung 850 EVO 500gb 2.5 inch Solid State Drive, now only around £100. You are basically looking at a new machine for £200 if you buy the best battery/drive combination. You've just saved two or three grand.
If your current set up is running slow, another solution is to do a clean wipe and install High Sierra as a new system, then reinstall the software you need. That usually fixes most slow downs that are not drive related.
I have a Macbook Pro 17" 2010, and I upgraded the drive and it works brilliantly. I don't see the need to constantly buy new computers if I can find ways to make the current machine work to a reasonable standard. The only things I'm missing out on that I could do with is faster rendering, 4K video editing and fast in/out data storage. Firewire 800 is a drag, but I can compensate rendering and data transfer by running overnight jobs. Meanwhile, I've got a lovely anti-glare 17" screen and a great keyboard that should keep me going for another two years. Ten years with one laptop is a great return.