I have a 2011 MacBook Air that is still running well, so I admire your efforts here, but as ku4hx indicates, you have an obsolete model. Apple won't touch it but some Apple Authorized Service Providers will; however they will try to talk you out of any such repairs because they are so expensive and original parts can't be found.
10.13.6 is an ancient MacOS, there are many things you can't do with it, and the connections on your 2011 MacBook Pro are (a) slow USB and (b) slow firewire. I don't think firewire drives are even made anymore. Your laptop also has just 802.11n, which is far behind the state of the art. The SSD is probably close to end of life (on my 2011 laptop the SSD fails the DriveDX and Techtool Pro SMART tests).
I am still using my 2011 laptop because it still works apparently well, but I don't trust it for anything important and as soon as something wears out, it will be retired.
TurboTax requires Big Sur minimum; Garmin devices can only utilize Garmin's software if you have 10.15 or later. Many web sites won't work anymore with 10.13.6 Safari (but many still work with Firefox and Chrome under 10.13.6), and only an obsolete version of MS-Office runs under 10.13.6 now. Might be time to start saving up for a new Mac!