It's worth making sure you are only using wifi, e.g. mobile data is disabled and bluetooth off. Although mobile data and bluetooth bridging/tethering should be faster than your minimum it's not a strong observation if there is potentially more than one path the data might take at any point in the test. You can turn bluetooth and mobile data back on after testing with them off. Though, if it helped to have both off it's worth testing again once with bluetooth off, mobile data on and once with bluetooth on, mobile data off. If either of the latter two tests is the only one with the same problem then you probably have either a low mobile data rate or a bridge AP (bluetooth or wifi) that's the cause. With Android devices that could just be another Android device. You can best disable bridging in the settings of the device that is the bridge connection point rather than your phone. It's in the Android Connections settings named Tethering I believe.
You should be able to see the wifi signal strength from your access point on the phone but it doesn't say whether it becomes inaccessible due to something like over-use of limited wifi channels at your location. Even in my home with no connecting walls to any other home I know all 2.4GHz and 5GHz channels are always in-use by neighbors and myself. The easiest way to spot that is to open the phone Settings appp and choose Connections->Wi-Fi. It should keep updating the list of avilable SSIDs in-range, look to see if yours sometimes disappears from the list. However, a more visual view of heavy channel use can be easily seen with an app called wifi analyzer, you can get it free on the Google's Play Store. I have no relationship to it, I just use it for the type of problem you describe. Although the free mode has ads they are not intrusive. You can click on the band list (2.4, 5, 6) at the top to toggle through them. What you should see is the channel usage per-SSID your phone can detect. The problem case will be if there are multiple overlapping SSIDs covering much of your own AP's SSID. There's not much you can do about other that than re-position your AP so that it has less interference, negotiate with your neighbors to all do the same or switch to a wifi 6e or wifi 7 AP that uses the GHz band where there are currently many more channels, though 6GHz won't reach as far within a home than 2.4GHz or 5GHz and you may have to look at a mesh or bridge system of two or three APs to improve range.
If you are familiar with configurating your wifi AP you might consider limiting the bit-rate on 5GHz channels, e.g. I supposedly have a 1Gbps Internet connection but congestion means it is rarely faster than 0.5Gbps (fast enough for streaming video) so instead of letting my APs use 2.4Gbps per-SSID I limited them to 0.6Gbps, perhaps 1.2Gbps would be better but the important thing is my SSIDs now have narrower bandwidth, higher peak signals that tend to be on the descending edge of interfering SSIDs so are easier to stay connected to. If your AP supports WPA2 and WPA3 security modes and you are using the mixed security mode WPA2/WPA3 but have no WPA2 devices (old devices or limited function devices like smart lightbulbs) then using WPA3 only can make connections more reliable (I would assume from your post WPA3 Personal mode specifically).
Variability of data throughput of such a large range is quite common with tcp based protocols, it's caused by something called congestion and there is little you can do about it on your phone. I experience it typically on weekdays after 5pm for a couple of hours, when my ISP's service gets busiest parents just home from work with phone and laptops, children on-line. If you can upgrade your service data rate it might help but I'd think of that as a very last resort approach.
