Aah yep, I also had a Das, and the Filco Majestouch 2. Beautiful keyboards. Sold 'em.
Now I use the very-average Microsoft Sculpt Ergonomic. Low keycaps, low travel. And of course a split layout.
Do not use the execrable MS mouse that comes with that keyboard. I love the MX Anywhere series, I currently use a 2 and a 3. They're light, so you can nudge them round with your fingers without having to move your arm too aggressively.
Ah yes I know that keyboard - mine is a Keychron K2 V2 with optical switches. It's decent to type on but I miss having a volume knob and proper function keys.
I find it really helps not having the number pad - it means my mouse can be closer to the keyboard so my wrists are straighter.
For a mouse I use a MX Master 3 which I'm very happy with.
I tried an MX Master 3 and 2S, I also have the MX Ergo trackball mouse and the Logitech vertical mouse. The vertical mouse is what I ended up with, but for years I would switch mice, switch between several mice within a day, switch hands - and they were all half-measures that just kind of delayed the "degeneration". The issue, it would seem, was not the mouse or overusing the mouse. The issue seems to have come from my shoulder area (Thoracic Outlet Syndrome aka TOS / Pec. Minor Syndrome), which a vertical mouse will uniquely address as it allows your shoulder to be less interiorly-rotated than a standard mouse, even a trackball mouse that is not vertical.
I remember reading a Medium article a while ago about a software engineer who described how he addressed his "RSI", which I suspect was just TOS - he switched to an ergonomic keyboard and vertical mouse, which over a period of MANY MONTHS more or less resolved his RSI. He talked about how he still has to be careful and take breaks often. I really think, especially considering his age (20s), he could have kept the mouse and keyboard he had, if he just engaged in posture-correcting shoulder stretches and strength exercises.
I tried the Master but found it a tad large. As above, I'm not a ginormous person: 172cm (5' 8", just), 70kgs. I don't have huge hands. I found pushing round the Master to be a bit too much work, which sounds preposterous but ¯\_(ツ)_/¯.
The Anywhere is just smaller, lighter. More nimble.
I also use an MX Master (2S), and wish I would have switched sooner. It can connect to 3 devices (via Bluetooth or USB receivers) with a button on the bottom to switch, and an “infinite” scroll wheel with multiple modes, which is awesome!
Now I use the very-average Microsoft Sculpt Ergonomic. Low keycaps, low travel. And of course a split layout.
Do not use the execrable MS mouse that comes with that keyboard. I love the MX Anywhere series, I currently use a 2 and a 3. They're light, so you can nudge them round with your fingers without having to move your arm too aggressively.