As someone who does have a background in this - RO is actually the simplest solution given that reducing lead is one of your worries and you want something for a rental. In fact, I'd say RO for the last stage even if you owned and had a multimedia filter in front of it. There is a lot of bullshit snakeoil out there as you've already noticed. Yes, they will waste some water, but given your description I am assuming you are on city taps anyhow rather than a well. If you are on a well and/or catchment like I am, you will want additional coarse filters in front of it to remove particulates. For tap water, just a simple three stage RO unit will do plenty fine. If you are worried about the cost of filters, you can get the prefilters much cheaper from an agricultural supply. Same ones, just in a plain box. Cheaper if you buy multiples and they don't go bad. These are cheap and important to keep changing. If you notice your water flow rate drop - change even if it is not your scheduled time. This will prolong the life of the more expensive RO replacements and give you the best filtration results.