• Philodendrons - first one that came to mind, so many varieties! The care has been really similar to monsteras for me.
• Hoya - agree with the other poster’s suggestion! I think a hoya would look nice next to your tradescantia, and there are so many interesting types. Plus they might flower if you’re lucky!
• ZZ plant - a bit of a different shape/growth pattern from what you have and quite low maintenance
• Peperomia - a more compact suggestion and somehow very giftable imo!

If he likes monsteras he'll love a rhaphidophora tetrasperma!

I think a jewel orchid such as a Macodes petola can make for a really nice gift for someone who likes plants. They are not in the "super easy" category but certainly "easy enough". There are some really cool varieties to choose from. Personally I love the ones with bright green and gold veins, but there are also dark colored ones.

Orchids do have some different requirements, so your boyfriend might have to do a bit of reading and video watching to learn about orchid care. You can get the orchid with some substrate, an orchid fertilizer, and a pot, and he'll be set. If you live in a dry place it might take some extra effort to keep it humid and happy, though.

I'm also a monstera and fern fan. I also like Aloes and Cycads. Cycads can be expensive and hard to find though. But aloes on the other hand, are much easier, and very easy to maintain.

Hindu/hoya rope plant would be a pretty cool addition to your collection.

If you have the space, a ficus is a pretty classic houseplant with different contours than the plants you have already.

Would you consider a carnivorous plant? I have a Butterworts plant and it helps with any little gnats or fruit flies that occasionally get inside.

They're relatively easy to propagate, unique and very satisfying to grow!