How do I improve my upload speed *without* calling my ISP?

You can try disconnecting the power from the actual ISP modem for a five minutes, then reconnect the power (can take 5 to 15 minutes to finish booting). Sometimes this will get you a different connection from the ISP. I'm referring to the modem or ONT which can sometimes be hidden at the point the ISP cable enters the building, and not the router (although power cycling the router is a good idea as well).

Otherwise at most you can check the cable & cable connections to the modem and routers. To bypass cable issues in the home you can try connecting the router with another cable to the modem. Or it may be possible to connect the Mac directly to the modem itself although this requires power cycling the modem & Mac in order to communicate directly with the ISP to get an IP address. With some modems a direct connection may not be possible (Verizon FIOS can be one which has no active Ethernet port on the modem/ONT depending on how the ISP has configured the device). Some cable modems it is possible to check the signal levels (Motorola is/was one of the few that allowed the user to do this), many block a user from checking the signal levels. Fiber connections you just rely on the status lights on the ONT.

It can be very difficult to resolve these types of issues if the issue is with the ISP. Some ISPs tend to overload their network nodes, or have bad connections down the road which is affected by the weather. If you can prove the issue occurs at the modem, then either the modem is the problem or it is an external issue only resolvable by the ISP. I gave up on my cable issues (some were outdoor neighborhood cable issue, and other issues were the ISP throttling & blocking traffic, and not updating nodes to handle all the traffic -- I detest that ISP with a passion) and was lucky enough to be able to switch to fiber which had recently made it to our neighborhood. Just keep in mind if the ISP sends someone to work/replace something inside the building, then the user will be charged a service fee. If the issue is outside, then the ISP is suppose to pay.

Maybe you can contact the ISP and have your father talk to them to authorize you to deal with the issue on your behalf, so you can deal with the technical details.

You should also boot your Mac into Safe Mode in order to prevent third party software from automatically launching during boot & login. If it works fine in Safe Mode, then it likely means you have some third party software installed that is affecting the normal operation of macOS.

sometimes the Router your ISP provides is a genuine antique. other times it could be mis-configured, or not set tup properly for your network neighborhood.

Wi-Fi is sometimes extremely crowded, resulting in low speeds. an apartment building is a good example.

If you are close to the Router or could run a cable, an Ethernet connection would get you great speeds up and down. for your specific MacBook Pro, you would need an adapter to use Ethernet.

one possible explanation of slow Upload speeds is that your upload connection is already saturated with traffic.

if you are running any third-party file sync-ers, such as OneDrive, google Drive, BackBlaze, or any cloud backup software, that can drag your upload speed to nothing. and it is not necessary or appropriate.

Just some food for thought: the ISP we used to have did have this in the fine print: Upload speed up to 1.0 mbps. It did not matter what the download speed was; unless you spent extra money on a different type of account, that was what they offered in a regular account.