I travel to Japan frequently and haven't bothered with getting a local SIM for years now. Unless you plan on making/receiving lots of calls, roaming these days is cheaper than buying a SIM card (I get 2 GB of data valid for 365 days across 90+ countries including Japan for a flat A$20 (~12 EUR) with Amaysim, your mileage may vary). You do not need a Japanese phone number to book trains, restaurants, or major hotels online.
On rare occasions, eg ryokan using some crappy locally developed reservation system, you run into a website that can't handle anything but Japanese numbers. (These sites are typically in Japanese only and insist on a Japanese address too.) For these I use a relative's contact info, but as far as I'm aware nobody has actually ever called her. And you could probably work around having to use the site in the first place by booking through a third-party aggregator like Rakuten or Jalan, which do support non-Japanese numbers.
All that said, if you do want to get a local number an e-SIM is the way to go these days, eg Sakura Mobile has plans starting from 2000 yen. Note that Japanese SIMs for visitors are data only unless you specifically choose to pay extra and jump through hoops to get a number.