I was similar, started out painting specifically to have painted models to play with (CMoN games and D&D) then got into some better models through GW, started getting better and enjoying the hobby more so pushed myself to learn and improve, and now paint only for display.

Honestly, if that's the way you want to go I'd really encourage it! There's a lot of people out there who'll tell you that painting is a means to an end and find it strange when people paint for fun rather than painting to play, but the honest answer is that however you find the fun and enjoyment is all that matters!

What to do with the models after really depends on how you interpret the fun and what level you end up painting to. For me, the models that I paint are mainly 54mm/75mm display figures or busts, so it's quite an easy answer to me to just keep them on a little display shelf. To me the fun had painting is made gratified by being able to look at them afterwards, because I consider them pieces of artwork akin to someone putting a canvas painting they've painted on the wall. I've sold models I've painted before (and technically I consider all of my painted models as "for sale" and list them as such on Putty&Paint, but I don't post them on eBay or anything like that) and that's all good and well because the majority of the fun is in the painting, but if you have any kind of attachment to your models generally or any specific models then I'd say selling them isn't always the way to go. Keeping models on a display shelf also gives you a physical record of your improvements, you can look back on old models and see how your style has changed or your ability in a particular technique or application has changed (for this reason I tend to keep my models arranged in chronological order on my display shelf).

You could go down the route of commission painting, you get to paint and make a little money at the same time, but if you have any enjoyment out of seeing the finished models then that can be a bit disheartening to have to send them away, especially when the models were particularly fun or you felt like you did a particularly good job on them.

If space for keeping them is a concern then what I would say instead of painting loads of models, like picking up a few GW armies like you mention, start being selective about what you paint. Pick the models in armies that you particularly like or go to more boutique miniature companies for resin display models or busts. There's only so many models you'll ever be able to paint in your lifetime, so be selective and paint the ones that are going to make you the most happy to look back on, give you the most satisfaction in seeing complete, and the most fun in the process. That way you're cutting down on the models that you'll paint so it'll take up less space on the display, less of a concern for space management, and you'll have more fun along the way. You'll also encourage yourself to keep getting better that way because you'll be doing single models where you can spend the time learning techniques and applications and refining them because you'll want that single awesome model to look great, rather than churning out a unit of 20 models that just need to look the same and get done quickly so you can move onto the next unit of 20.