I've been playing this one through the weekend, almost non-stop, fully free player, and so far, it appears to have almost infinite things to do.
While you do run out of energy doing the campaign, it has enough alternative activities to fill the gaps such as Tower, Conquest, Alternative Timelines and so on that so far I haven't had one single second of downtime where I've felt like I needed to either wait or buy energy refills to progress, not even using the option to buy energy refills with the ingame currency you're given for jus progressing through the game.
I'm pretty sure that will change at some point near the endgame, that's where things start getting steep, but the twist of this game is that being tactical combat, you as a player have a lot of agency to beat challenges that other similar games would be impossible. In most games, the combat basically comes down to your raw stats vs the enemy's raw stats, but in Sword of Convallaria you can be underpowered but through synergies and good tactical play get on top, and that feels pretty great.
Not a game for everyone of course, but it's free to play and so far, I can't say it's predatory to any extent that would make you feel like you need to keep paying to keep up. So far, having a lot of fun.