@Phatphoque98 Skipping rope comes with a bit of a learning curve. But a decent jump rope isn’t expensive and you can skip rope indoors assuming you have enough space (overhead clearance). Skipping rope can be done as a warm-up, HIIT (double-unders), and LISS. Like running, especially sprints, it could be argued that it is higher impact and doing a lot of it may interfere with your training. I skip rope for warm-ups on GPP days just for some variety. Skipping rope doesn’t involve the upper body musculature as much as an Airdyne, rower, etc. But I do feel a burn in my delts after a few minutes.
I second an Airdyne, Assault bike, Echo bike, or something similar. They would be well suited to warm-ups, HiIT, and LISS, less likely to interfere with training, and cost less than treadmills. They also have a relatively small footprint (don’t take up a lot of space), and are easier to move than a treadmill. Sometimes you can find a used one for sale on FB Marketplace, Craigslist, second-hand sporting goods stores, etc. A regular stationary bike would be ok, although it doesn’t incorporate the upper body musculature.
A rower isn’t a bad option. Price point is similar to an Echo bike. They have a pretty big footprint, but some can be folded and stowed out of the way. A plus for the rower is that it incorporates the abs, the shoulders, and upper back. It’s also relatively low impact. A minus is that LISS on a rower is boring, at least for me. I find a rower better suited for HIIT. Using a rower effectively requires greater concentration than a bike. I use a rower for warm-up on lifting days and for HIIT when the weather keeps me indoors. I’ll use it for LISS if I have to. I was shopping the used market in search of an Airdyne when I found a good deal on a rower.
Strongman variations for HIIT: I started incorporating these for variety. The primary argument against this is that it is relatively high impact/high stress. But I only HIIT once a week. I use a sandbag or a keg for carries. I also put the keg on an old car tire and pull it or drag it. Using a tire instead of a proper sled or prowler is quieter and won’t irritate the neighbors. Of course, I have to do these outside. You could also do sandbag over-the-shoulder pick-ups or sandbag-over-bar in the rack. I haven’t tried these yet. But it can be done indoors. I’ll probably use these this winter. Sandbags don’t have to be expensive. Although you’ll want a decent one or you might end up with sand everywhere. Sand is relatively cheap, and you can start with a small amount and add more as you adapt. I got the keg and car tire for free and had the tow rope. A 30 foot tow rope new isn’t expensive though.
As for treadmills, I can’t offer an informed opinion here as I haven’t done my homework on these. I prefer to jog outdoors for LISS when the weather cooperates. As a former cross country runner, I enjoy running. But I also have a few nagging little injuries that are aggravated by running. So I haven’t looked into a treadmill.
Hope something in all of that is helpful and can bridge the gap until you can get a treadmill. Good luck!