at the moment, i've created
field-X ( i also had an idea about checkboxes in dropdown made by css like
here , but still, with ~80options it isn't managable)
nice idea about tabs BTW, but it's all about personal preferences - I have used tabs from the early begining of project.
When form consists of 30 obligatory fields, some of these fields might be overlooked by user. And validation works not good with tabs - user have to search through all tabs (maybe 3,4,5..) again if faults
I also thought abot multi-steps, but this is not applicable, because of seblod mistakes and comlexity of the form. Very bad thing (as I've already tired to repeat) that Seblod is made
by developers for developers (by the way as many other joomla solutions).
To make frontend forms more user friendly, i have to add many custom JS code based on Jquery. As far as I'm not a programmer, it's difficult for me to make one nice JS and then call functions from that script in the field's settings (Stuff->custom JS). I can share it with you if you want of course ( i don't make money with development, it's my hobby :) )
Thus, modals in my opinion is the best way to bring user's attention to an action I need. The advantage of modal is such, that user come back to a form where he was, and can easily change selected options later. So he /she doen't get annoyed after redirects of seblod. ALL modern websites/social networks are trying to leave user on ONE page, especially if an important action is required. That's the philosophy i like...your page gets more reliability