Advice on varnishes

So I haven't actually painted with oils for a few years, but some recent research into varnishes has made me realise that varnishing oil paintings seems to be highly recommended. Although I studied Fine Art, I was left to my own devices when it came to painting (the course ended up being very contemporary), so I never received any formal training or education about the fundamentals of painting. I was aware of varnishes at the time, but had no experience of using them. I was always happy with the unvarnished finish of my paintings, and also reluctant to 'ruin' them by applying a varnish (I certainly wasn't keen on a gloss finish). Now I work in a very different medium, so I have a little more experience with varnishes. I make custom animal models from air dry clay, but these are finished in acrylic and sealed with varnish for both aesthetic and protective reasons, as the models are small and the clay is fragile.  So this comes to my question(s). Between 2015 - 2019 I sold 6 commissioned paintings, and sold 1 non-commission through a gallery showing. One of the commissions is framed under glass, and the gallery painting was also framed under glass (both with mount so the painting does not touch the glass).  All these paintings were painted in oils, with no varnish applied.   Now that I've recently learned more about varnishing oil paintings, I'm concerned that I should have completed this step on all my own paintings. I understand you should leave 6-12 months before varnishing, so this would have been an issue anyway for any commissioned work. I'm debating whether to contact all customers (all commissions are family & friends) to just let them know that the paintings are unvarnished and therefore have less protection against UV/dust/dirt, and that I could apply a layer of varnish if they would prefer, with an understanding that the painting may have a different 'look' due to the finish of the varnish.  Is this borderline excessive, or worth doing? I feel uneasy because people have paid money for these paintings and I'm concerned about the longevity without varnishes. I wanted to be upfront and transparent about my naivety with varnishes but also explain that I'm continuously learning as an artist. I'll upload some images of the artworks so you can see the style of the paintings - the work is predominantly realism with the exception of one. Apologies for the terrible quality, the quickest way to get them was from Instagram! I would appreciate some perspective on this from more experienced and educated artists than myself, I never know if I'm blowing something out of proportion or not! Thanks in advance.

Hi Robert - thanks so much for your reply. Actually my excessive googling of the subject brought me to this forum in a discussion about varnishes where you provided some very helpful info! I hoped you'd be able to help me here. It's good to know the glass is adequate protection. I don't believe I used either of those colours - I'll double check but neither of them look familiar at all to me. I actually generally paint in very thin layers with the addition of thinner (the Winsor & Newton oil thinner) with exception to one of the pieces.  If I were to add a varnish, I'd been looking at the Winsor & Newton range again - possibly the satin finish with UV protection. Does this seem like an adequate option for my situation? I have a painting sat in my studio from a few years ago that I'm thinking of using for varnish testing, so I'll probably try a couple of finishes on there. If there are any more you can recommend, I'll also try those too. I did come across the same information regarding dammar - and I agree I would prefer to stay away from that. Actually the yellowing of varnish was something that I remember had put me off originally, I just wish I'd done more research at the time. We live and learn! So it was actually a bachelors degree - I majored in Fine Art with English, but it turned out that the Fine art course had a large focus on contemporary art/sculpture, so none of the tutors seemed particularly interested in the more traditional methods of art. We butted heads quite a lot - I would have been better to go a different route with the education but again, we live and learn. But thank you for the compliment!