I agree, a good router will repay itself manifold over its life. You don't have to go for the biggest on the market, indeed a very small router is jolly useful, but certainly get one big enough for the job and choose quality over price. There is more advice on here about which router to buy than you can shake a stick at, just search the Buying Advice forum for Router and you are set for the next week's entertainment.
I would make solid oak (or WBP) splines fitted across the grain. Biscuits are a ver bad idea, DAMHIKT. Dowels are difficult unless you have a proper jig and add little to the strength of the joint, they only help with alignment, really. A groove and splines is a much better solution.
Glue is another consideration. I've not had much joy with polyurethanes, even though they are supposed to be waterproof. My last outdoor project I used Titebond 3. So far so good, although it is in a sheltered location and doesn't get rained on.
Welcome to the forum!
S