So you want rewards on a success yet no penalty for a failure. Is that fun for the other players? Why not just tell the DM you auto success?
Then being an equal or better than a fighter. Why would fighters exist if Thief could best them one vs one?
I found that the thief who goes solo then returns to the party has fun, but every other player at the table get very bored.
I agree..... but thats central to the whole archetype of what a theif IS. D&D has always had times when:
the bard having the fun charming a princess to get some vital info while the party lounges
the rogue is having fun, slicing throats while the party waits
the fighter is having fun, charging straight at the big boss, or intimidating weaklings in the tavern while the party supports and heals him
the wizard rains fire down on the enemy minions, while the party waits and stays away from the fireball
This is central to D&D. Sure there are times when thats annoying but its the DMs job to MANAGE it. There are sooo many ways to manage it. Maybe you invite the theif player to come an hour early one night, or maybe you tell the party, he guys, this is his moment to shine, have some nachos, dont worry theres some you moments soon. Or maybe you "timeslice" and make sure theres some thing interesting for everyone.
What you DONT DO is completely change a central tenant of the game by
making everyone equally useful at all times, and rejig the game based on assumptions that its a miniatures game that everyone can autoheal to stay in combat. We tried that it was ok, but it didn't feel like D&D to a lot of people.