If only one Bitcoin miner gets the block reward, is it worth mining only for the transaction fees as a (not-so-competitive) solo miner?
I will only be rewarded with the fees with respect to the calculations I have made.

That's not correct. If you don't mine a complete block, you get nothing at all.
Joining a pool will let you get a proportional share of the income (from both the block subsidy and the fees), changing the game from a lottery to a more predictable payout based on your contributed hashing power.