Step 1. Find Kick Sample. You already have them, so just pick one that works.

Step 2. Play a sine wave in the 30-60 Hz range, depending upon the key of your track. Every single DAW has a simple synthesizer that can do this, and most if not all DAWs have a visual indicator of frequency to note connection.

Step 3. Place slight saturation on that sine wave (optional, so it can show up better on laptop speakers)

Step 4. Put a compressor on top of the sine wave, sidechained to the signal of your kick so it ducks every time the kick hits.

I've been looking into bussing my sub bass and kick together for a low end submix anyways, so I'll try that. My main concern comes wth any variations in the kick pattern - I like to play with little kick fills and I want each thump to really cut through the mix.

don't over think it. its a sub, you want it to be clean and simple. i know nghtmare uses the "hip hop sub bass" preset in ableton's instrument rack for most of his subs.

just take a sin wave, saturate it and add a little distortion. If you want that booming kick, find a kick sample and layer it. I don't recommend ducking your sub around the kick because the attack is never perfect and you don't want your sub swooping in at some weird time signature, which will affect the grooves in your track whether you realize it or not. 2 options for this: 1) just let your kick hit on top of your sub and the distortion it creates wont be the end of the world and might sound cool on the track or 2) high pass your kick at 100 and give your sub patch a pitch envelope that matches the pitch envelope of your kick, probably starting 1 or 2 octaves up with a decay (or attack depending on how you use your envelopes) of 80-200 ms.

side note, a good way to mush your sub and kick together to sound more uniform is to filter out some of the high frequencies in the kick and then use the same saturator/distortion/whatever as your sub to give them similar tonal character.

Or you could download a hip hop 808 pack and use that. Don't kid yourself into thinking these guys design every element in their songs. Good songwriting is more important than good design. The people at ASDR or PrimeLoops have degrees in audio engineering, let them worry about designing instruments while you  focus on composition.

Ekali sample pack has those spacious Rustie sounding kicks. Those + an analog clip saturator with the gain boosted will give you what you're looking for. I remember reading in an interview that Rustie uses analog clip a ton. Also, I find it much easier to just synthesize an 808 rather than use a sample. Its really as simple as a sine wave + a bit of distortion/bit crushing or a lowpassed square wave. I usually do that in massive then throw on a saturator on medium or hard clip and low pass again to taste. That'll get you a super fat sub that you'll be able to hear on any speakers.

One last thing, you can sidechain your 808s to your kick if you want but I've had more success just highpassing the kick at like 150-200hz and just laying it over the 808. Depends on what I'm going for, if its a slower track then I sometimes go for that ducked sidechained sound but if it's a trap banger that's usually what I do