Lot of useful advice here for ya already, OP. Though you could get even better answers if you gave us some more insight into what you are trying to do.
Are you gaming? Is it for work? Is it for computer intensive stuff like video/3d/audio production? Is it for light use like web browsing and watching videos?
Do you want to sell your old computer or throw it away? Are you trying to keep it as a backup?
You don't have to get too personal, but a general idea of how much you want to spend would be good for getting you better tips.
Ok, I think I have a better idea of your situation now. So your computer is starting to feel a little weak when it comes to gaming? We've all been there before.
Depending on how easy/difficult it is for you to spend a couple grand, you might simply be best off buying a new PC at this point. If you're gonna keep the 55" TV hookup and your mouse/keyboard/controller/speakers, then you could find even a cheap prebuilt tower for under $1000 easily, and if you wanted to spend more, you could do that too if you want your build to stay relevant longer. Pretty much anything you might buy today, even stuff considered mid or entry level, will give you a noticeable improvement over your i3 and 1070.
Of course, the cheapest way to go would be to build the whole thing yourself, though it's entirely understandable if you just want to go prebuilt again so you don't have to worry about that stuff.
If you want to be more frugal, your best most effective single part upgrade would be a new SSD, preferably an NVME of good quality. Think in the $80-$200 range. If your motherboard doesn't have an NVME slot (possible, though unlikely) then a standard SSD is good too and would be a little cheaper.
For the rest of your system:
Upgrading your processor is probably not a cost effective solution. If you wanted a newer processor, you'd need to buy a new motherboard too (Google motherboard sockets for a full explanation). Depending on the type of motherboard, you might have to buy different memory too - I just upgraded to a new motherboard and I had to buy fancy (and expensive) new memory for it since my old DDR4 is no longer supported for it.
1070 is probably good for now, even though it is older. My 1080 handled BG3 just fine, as a point of reference. If you're ok with not playing at max resolution ultra super mega detail, just ride it out. GPU pricing has been effed since all the crypto bros made the prices skyrocket in 2021. Ride that 1070 until it dies, then buy something that is midrange today in a few years when it will be even cheaper.