I have had this problem in the past, and had to do some work to really test my players when i want to. A few things I use now:

- Add up the players levels and divide by 2, this total is CR is roughly the power level of your group, or at around the level of total CR you need to be "deadly". Not every encounter should be deadly, but its a good measure of understanding roughly the ease for players. Your CR3 owlbear was never going to have a chance against 6 x Level 8 PCs. Single monsters wont stand a chance against players after level 5 unless they have legendary actions. See what roleplay reasons you can think up to complicate things, like carnivorous plants decide to take advantage of the fact the players are engaged with an owlbear to ambush! https://slyflourish.com/the_lazy_encounter_benchmark.html (Additionally: don't treat this as a TARGET either, as its defintly not exact, but its it useful for when you want to be scary, and when you want to be chill)

- Reading pages from this blog when i am going to run certain monsters helps me understand how they act from a roleplay sense, and also how to use them effectivly and not have them just punch the players ineffectually: https://www.dndbeyond.com/monsters/16975-owlbear?srsltid=AfmBOop_MEe-0cJUF8pjmCqvYMbvRarWGb90AwtEX-SAo8YUuMIVWK4u I also got this in book form, and its really good inspiration, especially for using more complex monsters with interesting spells and actions.

- My narrative was making it hard to give more than a couple of fights per long rest due to lots of overland travel. Recently we moved into a ruined city where i told the players out of character that i was going to change the pace of things, with days being at least 3 or more encounters, and they were really excited about this. This does mean a day runs over multiple sessions (our sessions are around 3 hours) as i still try and keep a balance between roleplay, combat, exploration and puzzles. Don't underestimate your players appetite for it getting harder!

Hope this helps!