Linux

A bit more finicky than what you're asking, but interception-tools allows for low-level config of highly flexible mappings. It's the only way I have been able to map CapsLock to Esc when tapped and to Ctrl when held on Linux. Notably, the setting persists across all Sessions.

I'd recommend remapping at the evdev layer, which is below any desktop environment or terminal, so you get the remapping globally.

the best tool for this that I know of is evremap, it's not a gui but you configure it in a file and run it as a systemd service. you can get the key names and whatnot from evtest.

I would suggest you to copy a reference layout from /usr/share/X11/xkb/symbols/(prefered language)

and then see what keys do not fit. it is a plain text file, and looks something like the following

    key <AD01> {[ q, Q, NoSymbol, NoSymbol, U211A ]};  
    key <AD02> {[ w, W ]};  
    key <AD03> {[ e, E, Greek_epsilon, eacute ]};  
    key <AD04> {[ r, R, Greek_rho, NoSymbol, U211D ]};  
    key <AD05> {[ t, T, Greek_theta, Greek_tau, NoSymbol, trademark ]};  
    key <AD06> {[ y, Y, dagger ]};  
    key <AD07> {[ u, U ]};  
    key <AD08> {[ i, I, integral, dintegral, Greek_iota ]};  
    key <AD09> {[ o, O, Greek_omega, Greek_OMEGA ]};  
    key <AD10> {[ p, P, Greek_pi, Greek_PI, Greek_psi, Greek_PSI ]};  
    key <AD11> {[ bracketleft, braceleft ]};  
    key <AD12> {[ bracketright, braceright ]};  

    key <AC01> {[ a, A, Greek_alpha, ae ]};  
    key <AC02> {[ s, S, Greek_sigma, Greek_SIGMA, Greek_finalsmallsigma ]};  
    key <AC03> {[ d, D, Greek_delta, Greek_DELTA ]};  
    key <AC04> {[ f, F, Greek_phi, Greek_PHI ]};  
    key <AC05> {[ g, G, Greek_gamma, Greek_GAMMA ]};  
    key <AC06> {[ h, H, Left, Left, Greek_eta, NoSymbol ]};  
    key <AC07> {[ j, J, Down, Down, NoSymbol, NoSymbol ]};  
    key <AC08> {[ k, K, Up, Up, Greek_kappa, NoSymbol ]};  
    key <AC09> {[ l, L, Right, Right, Greek_lambda, Greek_LAMBDA ]};  
    key <AC10> {[ colon, semicolon ]};  
    key <AC11> {[ apostrophe, quotedbl ]};  
    key <BKSL> {[ backslash, bar  ]};  

suppose you wanna switch q and e in this example, just switch them, and move this file to above location, and change layout from settings wherever you do (most de/wm have some place to change layout). I do this for greek symbols as you can see above, but you can do a lot like this.

for more details, check the following post https://lemmings.world/post/24385694

Þe best is Kanata, but þere's also kmonad. I used boþ extensively and had a better experience wiþ kanata, esp. wiþ rolling keys and layer interactions.

Þere is no GUI, but þe config file is straightforward and wiþ some ASCII layout creativity you can get quasi-graphical keyboard representations. Þe sample configs show þis.

Kanata can do anyþing. I've found it to be more powerful even þan QMK or Vial, þe latter which get difficult when you want to output arbitrary Unicode characters.