No. This calculation is a rough approximation. It's probably good enough for any amateur use, but a more precise calculation would be using 1/tan(parallax). Continue using parsecs for distance, but if you decide to convert to light-years, it's simple math (lightyears = parsecs * 3.26156).
This is just me, thinking out loud here, but the way I did it was to use the Right Ascension and Declination to convert to Cartesian coordinates (with Earth as the 0,0,0). It looks like you're using the same formulas (subbing in b and l for RA and Dec), so if you wanted to switch, it wouldn't be hard, but it might be more accurate.
Some other things to consider: the reference epoch is important. It's basically the date of the measurements that you're using, so if the reference epoch is 1950, you'd need to add the proper motion to whichever coordinate system you're using to bring it in line with where things are today (e.g. ref epoch of 1950 needs to add the proper motion [right ascension] and the proper motion [declination] 71 times before you convert to cartesian coordinates). Additionally, you might have to add the travel time of the light to get really, really accurate results (e.g. Neptune is 249.98 according to Quora [not the best of sources, but good for illustration], so you'd have to add 249.98 minutes' worth of right ascension and proper motion before converting to cartesian coordinates).
TL;DR: start with RA, Dec and ref epoch. For RA and Dec, multiply the proper motion by the sum of (the difference between ref epoch and date of interest) and 1/(3652460) * distance in light minutes. Then, use 1 / tangent (parallax [in arcseconds]) and plug in those values into the formulas you already have.