This is not saying they wouldn't bring up these similarities if they were trying to shut you down for things they have firmer grounds for, just that amongst all the panoply of games and media using these D&D concepts (along with a few other things that have gained broader cultural impact like, say, the alignment system), singling out you for a lawsuit on the basis of things they have a pretty questionable case on makes very little sense.
The specific 5e action economy, however, is pretty unique to that specific D&D system and has not really filtered out into general use. A similar system with different names is almost certainly fine because on a basic level it is pretty generic to the limited number of ways that turned-based tabletop gaming could simulate combat and not very different from some other systems, but adopting their system wholesale is asking for trouble. If a game that was also using the ability scores and some of the same classes, also used literally the same action economy that is the sort of thing WotC is likely to identify as an infringement they need to protect against. And honestly, I like the 5e system well enough for the most part but I'm betting you could do better having years of experience with it and not being beholden to maintaining it like the current WotC design team.
Spells are troublesome because some are clearly very generic (Invisibility), some were perhaps less generic when D&D adopted them, but at this point you would hardly have a game involving magic without (Fireball) some that are fairly fanciful and specific (Magnificent Mansion, even if it doesn't belong to Mordenkainen), and one that is pretty specific but that they stole from Jack Vance (Prismatic Spray, though they diminished the theft somewhat by watering it down from his superior "Excellent Prismatic Spray"). That analysis is really more on the trademark merits than the copyright merits, but the two areas of intellectual property law blend together when you get into a situation like this.
Ultimately I would err on the side of using those game elements where, upon giving it serious thought you honestly can't think of anything that suits your needs better, and making the rest up yourself. But copying the ability score system outright (if you like it) might be worth the (probably) minimal risk, since it makes your game substantially more compatible with the vast number of existing D&D and derivative modules.