Warframe, like any game, has limited content. It is still being updated, but not NEARLY at the pace you two are going through it.
To put it bluntly, you’re going to run out of things to do before too long if all you do is level stuff and clear missions all day. It’s not a triple-A game with hours of side quests and character development or story. We’ve got 2 cinematic quests, half of a dozen 30-minute “go here and do a thing” quests, and some planets to clear. Excluding leveling stuff, it’s something you could do in a day and have time left over.
The vast majority of Warframe’s gameplay is gathering materials to make weapons, then leveling said weapons. Once you go through those, there’s depressingly little to do and you’ll get bored of it.
An easy way to slow progress is to only play solo. No public matchmaking, no clan runs, just by yourselves (of course, stay in your clan for access to research, just don’t go piggyback missions with them). That way you can’t take something into a mission and accidentally fully level it without ever having used it. Forces a bit of artificial slowness and makes it last a bit longer.
Now, I actually really enjoy Warframe’s gameplay. I could play it for hours without getting bored, but I don’t know if you and Tsathy can, given how many other games you two have and get burnt out on.
You may wonder why I have so many thoughts about the subject, but it’s mostly because I’ve made a lot of friends through that game, and so many have stopped playing so I’ve lost contact with them because of it. They got burned out because they blasted through ranks and quickly found out how bare Warframe is without the presence of items to level constantly. I don’t want you two to just drop it because it didn’t have enough for you two to burn through. Want you to get every possible bit of enjoyment out of it.