Reddit mentions of GEEKOTO 79 inches Carbon Fiber Camera Tripod Monopod with 360 Degree Ball Head 1/4 inch Quick Shoe Plate Professional Tripod Load up to 26.5 pounds

Under $350, you really can't do much in the way of upgrading to a better telephoto lens, so I'd stick with the 70-300.

The 18-55 is good for landscapes. What you really need is a tripod for it. You can get a great one for about $150 these days, and the monopod part would help with sports, probably. But the real trick to landscape photography is actually not about the camera or the lens, since you're usually stopping down the lens to f/8 or f/11 and so even a mediocre lens will give you good images. The trick is that it takes a lot of discipline, mainly in getting up early or staying out late, because the few hours after sunrise and the few hours before sunset give you the best light, that is, the "golden hours". There are also the "blue hours" immediately before sunrise and after sunset. The second part of the discipline, besides the timing, is the repetition. You may have an awesome shot, but then it's cloudy, or the light isn't right, or whatever. Some of the great landscape photographers visit a spot dozens of times before they get "the shot". A lens, a camera, and a tripod, and lots of discipline.

The 35/1.8 AF-S DX is a good lens to start with and you can pick one up used for around $120. You can also get a 50/1.8 AF-S for about $150 used, or $220 new, which is a great portrait lens on your camera. These lenses let in much more light (about 8x as much as your 18-55 does at 35 and 50mm) and also allow you to create more blurred backgrounds. I like the 50 much better than the 35 for portraits; for me the 35 is too wide to be flattering unless you're doing an environmental portrait and including a good deal of the room/environment around the person (and if that's the case, just use your 18-55, since you'll want more depth of field (less background blur) to include the details of the environment.)

So yeah, if it were me, I'd get a good tripod/monopod like the link above for landscapes, and the 50/1.8 AF-S for portraits. That's about $300 right there if you get the lens used; there's tons of them on eBay or if eBay scares you, KEH has them in EX+ condition for $150 too. Buying lenses new is one of the biggest wastes of money you can do in photography (and it was a lesson I didn't really learn until I'd spent thousands!)

Here are a couple, the first link is the one pictured. On this trip on of the springs for the legs broke, but the tripod still functions 100%

https://www.amazon.com/dp/B07H7GH9HR/ref=cm_sw_r_cp_api_i_YJ4IDb1AYRPK4

https://www.amazon.com/dp/B07DMMDYM3/ref=cm_sw_r_cp_api_i_FK4IDb1VVKVH3

https://www.amazon.com/dp/B07QM926M3/ref=cm_sw_r_cp_api_i_AL4IDbDBYQS4P

Is this a decent tripod? https://www.amazon.com/Geekoto-inches-Monopod-Professional-CT25Pro/dp/B07H7GH9HR/ref=pd_sbs_421_4/142-6169695-4800047?_encoding=UTF8&pd_rd_i=B07H7GH9HR&pd_rd_r=a8b335e8-ba26-4508-ad6b-98a279b29835&pd_rd_w=V1Jtj&pd_rd_wg=Ak4TY&pf_rd_p=1c11b7ff-9ffb-4ba6-8036-be1b0afa79bb&pf_rd_r=SX8Q13J2HHKM9JMSHDGT&psc=1&refRID=SX8Q13J2HHKM9JMSHDGT