Yeah I wouldn't worry too much about the length of Solasta. In my experience, if the content is of a high quality, you end up spending way more time with the game than what is estimated. Plus, yes there will surely be at least one expansion given how successful their KS ended up being.
Having said this, and I am a backer of Solasta, I am way more excited for Realms Beyond than Solasta. RB is using the 3.5e D&D OGL, which for me is the definitive edition of D&D. It has a party size of six, which is a major factor for me. It uses hexes rather than squares for its gridmap. And even though also TB, the devs have stated that they appreciate that there are many players who prefer RTwP and keeping that in mind are incorporating elements into their combat system to make it move more quickly and to try and make it less tedious and boring for the RTwP-preferring folks. For me, that kind of thoughtfulness and consideration is meaningful. So, all in all, RB is the better game for me.
But, there are a couple of additional games out there as well, that are inspired by the IE games even if they're not D&D-based games, which I am very excited about and looking forward to: Black Geyser: Couriers of Darkness, and The Dark Eye: Book of Heroes. The HUGE plus of these two games for me is that they are RTwP. It's also why I love Pathfinder: Kingmaker, and am looking forward to Owlcat's upcoming second Pathfinder game announcement, though keeping my fingers crossed that they won't bow to the "All games MUST be TB" coercion.
I am no longer wedded to a game needing to be D&D-based. I actually see the D&D RPG system, and especially the D&D combat system as being well short of ideal and having a ton of room for and need of improvement. The only thing D&D that continues to interest me and which matters to me is the Forgotten Realms setting, because I really love that setting. As such, I wish there were an OGL for the FR setting, so that someone could just use that setting but NOT the D&D ruleset to make a game.