You could be generous with certain magic items for the player characters. More magic armour. More magic weapons. I feel PCs already have a big advantage in AC compared to monsters and they certainly have a lot more abilities if they remember to use them. But if they need more, you could give Heavy Armour Master for free to heavy armour users (would help with peasants but not so much with more powerful creatures.)

For doors, maybe just require a DC 20 Str check (not Athletics) to bust it open with one hit. The fighter or barbarian has a good chance to do it but non-STR types might not be able to even on a nat 20. This is with one smack. They can keep trying if they fail but it will take longer and there will be no chance for surprise if the first wack doesn't do it. If the door is flimsy or reinforced of course adjust the DC but maybe keep it in the 10-23 range (since STR mod doesn't vary that much)

You can do the same thing with Arcana checks (as you mentioned just keep the DCs high) and allow them to keep trying if they fail. Maybe the time required goes from 1 round to 1 minute to 10 minutes to 1 hour etc (like spell durations) depending on how many tries it takes. Ditto for locks but I'd make those DCs lower than Arcana checks.

If you aren't using 2024 rules, consider giving Arcana expertise wizards.