Is there a (Windows) application that takes multiple, consecutive tracks, and merge them into a single GPX file?
I tried deadware "GPS Track Editor", but neither Merge nor Join did the trick:
I would use GPS Babel - you can download it free here: https://www.gpsbabel.org/download.html. It's a great, simple GUI that will take any number of GPX file (or any other types that are associate with random GPS collections) and merge them into one GPX, or any other format you want to output. You can also select whether you want just Waypoints, Routes and Tracks, or everything.
I came across this How can I merge two GPX files into one?, it should work for you.
GPX files are XML files, which means you can edit them in a text editor. I use Notepad++ on Windows and TextWrangler on OSX, but you should be able to use the built-in Notepad or TextEdit. Don't use a word processor like Word or OpenOffice.
Open the second GPX file in a text editor. Copy all the stuff between and inclusive. Open the first GPX file in a text editor. Paste the copied stuff after the line near the end of the first file and save. If you have more files, repeat.)
A GUI option GPS Visualizer: Do-It-Yourself Mapping
GPS Visualizer is an online utility that creates maps and profiles from geographic data. It is free and easy to use, yet powerful and extremely customizable. Input can be in the form of GPS data (tracks and waypoints), driving routes, street addresses, or simple coordinates. Use it to see where you've been, plan where you're going, or quickly visualize geographic data (scientific observations, events, business locations, customers, real estate, geotagged photos, etc.).
You can try this online tool: GPX Weaver - Simple GPX Editor
You can upload multiple GPX files, merge and delete tracks and segments, as well as change track color for BMW Navigators.